ref-manual: add uboot-extlinux-config class documentation

diff --git a/documentation/ref-manual/classes.rst b/documentation/ref-manual/classes.rst
index 98dff1bac..9fbfc0b2f 100644
--- a/documentation/ref-manual/classes.rst
+++ b/documentation/ref-manual/classes.rst
@@ -3470,6 +3470,61 @@  possible.
 See the :term:`UBOOT_CONFIG` and :term:`UBOOT_MACHINE` variables for additional
 information.
 
+.. _ref-classes-uboot-extlinux-config:
+
+``uboot-extlinux-config``
+=========================
+
+The :ref:`ref-classes-uboot-extlinux-config` class provides support for
+generating an ``extlinux.conf`` file part of the `Boot Loader Specification
+<https://uapi-group.org/specifications/specs/boot_loader_specification/>`__.
+
+The class can be inherited in a `U-Boot <https://u-boot-project.org/>`__ recipe
+with::
+
+   inherit uboot-extlinux-config
+
+However, the class functionality is disabled by default. To enable it, set the
+:term:`UBOOT_EXTLINUX` variable to "1" from the U-Boot recipe or from a
+``bbappend`` file::
+
+   UBOOT_EXTLINUX = "1"
+
+In addition to :term:`UBOOT_EXTLINUX`, the :term:`UBOOT_EXTLINUX_ROOT` variable
+must be set to the ``root=`` parameter of the `Linux kernel command-line
+<https://docs.kernel.org/admin-guide/kernel-parameters.html>`__. For example,
+the following would set this value to the second partition of MMC block device
+0::
+
+   UBOOT_EXTLINUX_ROOT = "root=/dev/mmcblk0p2"
+
+After building the U-Boot recipe, an ``extlinux.conf`` is generated and placed
+in the deployment directory (:term:`DEPLOY_DIR_IMAGE`):
+
+.. code-block:: text
+
+   tmp/deploy/images/<MACHINE>/extlinux.conf
+
+This file can be customized using the variables beginning with
+``UBOOT_EXTLINUX_`` in the :doc:`Variables Glossary </ref-manual/variables>`
+section of the Yocto Project Reference Manual.
+
+.. note::
+
+   Multiple ``LABELS`` can be specified through the
+   :term:`UBOOT_EXTLINUX_LABELS` variable (i.e. multiple boot entries).
+   Override-style assignment should then be used to specify label-specific
+   properties. See the definition of :term:`UBOOT_EXTLINUX_LABELS` for more
+   information.
+
+.. tip::
+
+   When using the :ref:`bootloader <ref-manual/kickstart:Command: bootloader>`
+   command with WIC, you should explicitly configure the ``.wks.in`` file to use
+   the ``extlinux.conf`` file generated by this class with
+   ``--configfile="${DEPLOY_DIR_IMAGE}/extlinux.conf"``. Otherwise it generates
+   a default ``extlinux.conf`` file without taking this class into account.
+
 .. _ref-classes-uboot-sign:
 
 ``uboot-sign``
diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index e75f42afe..ccd864877 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -11570,6 +11570,141 @@  system and gives an overview of their function and contents.
       The default is ``txt`` which means the script is installed as-is, with
       no modification.
 
+   :term:`UBOOT_EXTLINUX`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class in a
+      U-Boot recipe, the :term:`UBOOT_EXTLINUX` variable should be set to "1" to
+      enable the class functionality (inheriting the class is not enough).
+
+   :term:`UBOOT_EXTLINUX_CONSOLE`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_CONSOLE` variable can be set to control the
+      ``console=`` parameter of the `Linux kernel command-line
+      <https://docs.kernel.org/admin-guide/kernel-parameters.html>`__. For
+      example, use as follows::
+
+         UBOOT_EXTLINUX_CONSOLE = "console=ttyS0,115200n8"
+
+      This is added to the ``APPEND`` property of the ``extlinux.conf`` file
+      used for booting.
+
+   :term:`UBOOT_EXTLINUX_FDT`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_FDT` variable can be set to control the ``FDT``
+      property of the ``extlinux.conf`` file used for booting, which controls
+      the Linux kernel device tree to use. For example, use as follows::
+
+         UBOOT_EXTLINUX_FDT = "../am335x-bone.dtb"
+
+      .. note::
+
+         This path is relative to the ``extlinux.conf`` file used for booting.
+
+   :term:`UBOOT_EXTLINUX_FDTDIR`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_FDTDIR` variable can be set to control the ``FDTDIR``
+      property of the ``extlinux.conf`` file used for booting, which controls
+      the directory location for locating and loading the Linux kernel device
+      tree specified in the ``fdtfile`` U-Boot environment variable. For
+      example, use as follows::
+
+         UBOOT_EXTLINUX_FDTDIR = "../"
+
+      .. note::
+
+         This path is relative to the ``extlinux.conf`` file used for booting.
+
+   :term:`UBOOT_EXTLINUX_FDTOVERLAYS`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_FDT` variable can be set to control the ``FDTOVERLAYS``
+      property of the ``extlinux.conf`` file used for booting, which controls
+      the Linux kernel device overlays tree to apply on top of the device tree.
+      For example, use as follows::
+
+         UBOOT_EXTLINUX_FDTOVERLAYS = "../am335x-bone-wifi.dtbo"
+
+      .. note::
+
+         This path is relative to the ``extlinux.conf`` file used for booting.
+
+   :term:`UBOOT_EXTLINUX_KERNEL_ARGS`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_KERNEL_ARGS` variable can be used to pass extra
+      parameters to add to the `Linux kernel command-line
+      <https://docs.kernel.org/admin-guide/kernel-parameters.html>`__. For
+      example, use as follows::
+
+         UBOOT_EXTLINUX_KERNEL_ARGS = "rootwait ro"
+
+      This is included in the ``APPEND`` property of the ``extlinux.conf`` file
+      used for booting.
+
+   :term:`UBOOT_EXTLINUX_KERNEL_IMAGE`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_KERNEL_IMAGE` variable can be used to specify the
+      ``KERNEL`` property of the ``extlinux.conf`` file used for booting, which controls
+      the Linux kernel image to use. For example, use as follows::
+
+         UBOOT_EXTLINUX_KERNEL_IMAGE = "../zImage"
+
+      .. note::
+
+         This path is relative to the ``extlinux.conf`` file used for booting.
+
+      .. tip::
+
+         The :term:`KERNEL_IMAGETYPE` variable can be used to get the name of
+         the current Linux kernel being built by its associated recipe.
+
+   :term:`UBOOT_EXTLINUX_LABELS`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_LABELS` variable contains a list of labels
+      (``LABEL`` property of the ``extlinux.conf`` file used for booting) that
+      can be used for booting. This list should contain at least one entry.
+
+      When multiple labels are specified in this list, all of the variable
+      ``UBOOT_EXTLINUX_`` variable should be specified with overrides-style
+      assignments. For example, if the :term:`UBOOT_EXTLINUX_LABELS` variable
+      contains::
+
+         UBOOT_EXTLINUX_LABELS = "compressed uncompressed"
+
+      Then the :term:`UBOOT_EXTLINUX_KERNEL_IMAGE` variable can be specified
+      multiple times as follows::
+
+         UBOOT_EXTLINUX_KERNEL_IMAGE:compressed = "../zImage"
+         UBOOT_EXTLINUX_KERNEL_IMAGE:uncompressed = "../Image"
+
+      This will make the value of the ``KERNEL`` property be different in each
+      of the associated labels.
+
+      Note that default values are used when no overrides-style assignments are
+      found for the current label. For example, taking the above example again,
+      the following assignment would apply to both ``compressed`` and
+      ``uncompressed`` labels::
+
+         UBOOT_EXTLINUX_FDT = "../am335x-bone-wifi.dtbo"
+
+   :term:`UBOOT_EXTLINUX_MENU_DESCRIPTION`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_MENU_DESCRIPTION` variable sets a description for
+      the label. If not specified, the name of the label itself is used as the
+      description.
+
+   :term:`UBOOT_EXTLINUX_MENU_TITLE`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_MENU_TITLE` variable sets the ``MENU TITLE``
+      property of the ``extlinux.conf`` file used for booting.
+
+   :term:`UBOOT_EXTLINUX_ROOT`
+      When inheriting the :ref:`ref-classes-uboot-extlinux-config` class, the
+      :term:`UBOOT_EXTLINUX_ROOT` variable is mandatory and should contain the
+      ``root=`` parameter of the `Linux kernel command-line
+      <https://docs.kernel.org/admin-guide/kernel-parameters.html>`__. For
+      example, the following would set this value to instruct the kernel to use
+      the second partition of MMC block device 0 as its root partition::
+
+         UBOOT_EXTLINUX_ROOT = "root=/dev/mmcblk0p2"
+
    :term:`UBOOT_FIT_ADDRESS_CELLS`
       Specifies the value of the ``#address-cells`` value for the
       description of the U-Boot FIT image.