[7/7] runfvp: add basic documentation

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+# Running Images with a FVP
+
+The `runfvp` tool in meta-arm makes it easy to run Yocto Project disk images inside a [Fixed Virtual Platform (FVP)][FVP].  Some FVPs, such as the [Arm Architecture Models][AEM], are available free to download, but others need registration or are only available commercially.  The `fvp-base` machine in meta-arm-bsp uses one of these AEM models.
+
+## Running images with `runfvp`
+
+To build images with the FVP integration, the `fvpboot` class needs to be inherited.  If the machine does not do this explicitly it can be done in `local.conf`:
+
+```
+INHERIT += "fvpboot"
+```
+
+The class will download the correct FVP and write a `.fvpconf` configuration file when an image is built.
+
+To run an image in a FVP, pass either a machine name or a `.fvpconf` path to `runfvp`.
+
+```
+$ ./meta-arm/scripts/runfvp tmp/deploy/images/fvp-base/core-image-minimal-fvp-base.fvpconf
+```
+
+When a machine name is passed, `runfvp` will start the latest image that has been built for that machine. This requires that the BitBake environment has been initialized (using `oe-init-build-env` or similar) as it will start BitBake to determine where the images are.
+
+```
+$ ./meta-arm/scripts/runfvp fvp-base
+```
+
+Note that currently meta-arm's `scripts` directory isn't in `PATH`, so a full path needs to be used.
+
+`runfvp` will automatically start terminals connected to each of the serial ports that the machine specifies.  This can be controlled by using the `--terminals` option, for example `--terminals=none` will mean no terminals are started, and `--terminals=tmux` will start the terminals in [`tmux`][tmux] sessions.  Alternatively, passing `--console` will connect the serial port directly to the current session, without needing to open further windows.
+
+The default terminal can also be configured by writing a [INI-style][INI] configuration file to `~/.config/runfvp.conf`:
+
+```
+[RunFVP]
+Terminal=tmux
+```
+
+Arbitrary options can be passed directly to the FVP by specifying them after a double dash, for example this will list all of the FVP parameters:
+
+```
+$ runfvp fvp-base -- --list-params
+```
+
+## Configuring machines with `fvpboot`
+
+To configure a machine so that it can be ran inside `runfvp`, a number of variables need to be set in the machine configuration file (such as `meta-arm-bsp/conf/machine/fvp-base.conf`).
+
+Note that at present these variables are not stable and their behaviour may be changed in the future.
+
+### `FVP_EXE`
+
+The name of the FVP binary itself, for example `fvp-base` uses `FVP_Base_RevC-2xAEMvA`.
+
+### `FVP_PROVIDER`
+
+The name of the recipe that provides the FVP executable set in `FVP_EXE`, for example `fvp-base` uses `fvp-base-a-aem-native`.  This *must* be a `-native` recipe as the binary will be executed on the build host.
+
+There are recipes for common FVPs in meta-arm already, and writing new recipes is trivial.  For FVPs which are free to download `fvp-base-a-aem.bb` is a good example. Some FVPs must be downloaded separately as they need an account on Arm's website, `fvp-base-r-aem.bb` is a good example of those.
+
+If `FVP_PROVIDER` is not set then it is assumed that `FVP_EXE` is installed on the host already.
+
+### `FVP_CONFIG`
+
+Parameters passed to the FVP with the `--parameter`/`-C` option.  These are expressed as variable flags so individual parameters can be altered easily. For example:
+
+```
+FVP_CONFIG[bp.flashloader0.fname] = "${DEPLOY_DIR_IMAGE}/fip-fvp.bin"
+```
+
+### `FVP_DATA`
+
+Specify raw data to load at the specified address, passed to the FVP with the `--data` option.  This is a space-separated list of parameters in the format `[INST=]FILE@[MEMSPACE:]ADDRESS`. For example:
+
+```
+FVP_DATA = "cluster0.cpu0=${DEPLOY_DIR_IMAGE}/Image@0x80080000 \
+            cluster0.cpu0=${DEPLOY_DIR_IMAGE}/fvp-base-revc.dtb@0x83000000"
+```
+
+### `FVP_APPLICATIONS`
+
+Applications to load on the cores, passed to the FVP with the `--application` option.  These are expressed as variable flags with the flag name being the instance and flag value the filename, for example:
+
+```
+FVP_APPLICATIONS[cluster0] = "${DEPLOY_DIR_IMAGE}/linux-system.axf"
+```
+
+Note that symbols are not allowed in flag names, so if you need to use a wildcard in the instance then you'll need to use `FVP_EXTRA_ARGS` and `--application` directly.
+
+### `FVP_TERMINALS`
+
+Map hardware serial ports to abstract names. For example the `FVP_Base_RevC-2xAEMvA` FVP exposes four serial ports, `terminal_0` to `terminal_3`.  Typically only `terminal_0` is used in the `fvp-base` machine so this can be named `"Console"` and the others `""`.  When runfvp starts terminals it will only start named serial ports, so instead of opening four windows where only one is useful, it will only open one.
+
+For example:
+```
+FVP_TERMINALS[bp.terminal_0] = "Console"
+FVP_TERMINALS[bp.terminal_1] = ""
+FVP_TERMINALS[bp.terminal_2] = ""
+FVP_TERMINALS[bp.terminal_3] = ""
+```
+
+### `FVP_CONSOLE`
+
+This specifies what serial port is used when `--console` is passed to runfvp. Note that this has to be the FVP identifier but without the board prefix, for example:
+```
+FVP_CONSOLE = "terminal_0"
+```
+
+### `FVP_EXTRA_ARGS`
+
+Arbitrary extra arguments that are passed directly to the FVP.  For example:
+
+```
+FVP_EXTRA_ARGS = "--simlimit 60"
+```
+
+
+[AEM]: https://developer.arm.com/tools-and-software/simulation-models/fixed-virtual-platforms/arm-ecosystem-models
+[FVP]: https://developer.arm.com/tools-and-software/simulation-models/fixed-virtual-platforms
+[tmux]: https://tmux.github.io/
+[INI]: https://docs.python.org/3/library/configparser.html