[meta-darwin,0/1] macOS DMG/PKG

This patch adds support for building both macOS .dmg and .pkg installer files.

DMG disk images are the most common way to distribute and use macOS software.
Recent versions of Xcode distribute the internal platform SDKs as .dmgs and
then Xcode mounts them internally.  This makes it fairly easy to have multiple
toolchains and sysroots without having to manage a lot of installed files.
There is one main obstacle, and a few details to sort out.  The main obstacle
is that running unsigned binaries on macOS is becoming more and more
difficult.  It's possible to configure a system allow unsigned binaries, but
it's a big security risk and generally not a good idea.  As far as I know,
it's only possible to sign binaries on a macOS system.  So here we are stuck
with running something on the Mac post-install.  The other main detail is how
to relocate the installed files.  The Yocto SDK is designed to be re-located
at install time - with the expectation that there is no way to know the
installed path until the user installing it picks it.  With a .dmg image, this
is more managable at SDK build time, but since we are already running
something on the Mac to sign binaries, we might as well do some relocating
here and remain somewhat aligned with the standard Linux SDK.

The older, but still supported way of installing software on a Mac is through
the .pkg installer files. This is more or less a set of xml files, scripts,
and archives that allow the system Installer.app to install applications or
data on the system.  It does provide a convenient way to install directly to
any macOS volume, or to the user's home directory.  It will appropriately
elevate permissions so that there is no need to explicitly use `sudo` or ask
for a password.  Since we are able to run pre-install and post-install
scripts, the .pkg builder sets it up to run the code signing and relocations
scripts during the install.

To give the developers some control over the darwin SDK, we have the following variables:

DARWIN_INSTALLER_PKG = "1"
DARWIN_INSTALLER_DMG = "1"
DARWIN_INSTALLER_SH = "0"
DARWIN_INSTALLER_PATH = "/Library/Developer/org.openembedded.sdk"

Setting DARWIN_INSTALLER_PKG to "1" will enable building of the .pkg
installer.  Setting DARWIN_INSTALLER_DMG to "1" will enable building the .dmg
file.  If we are building the .dmg or the .pkg (or both), we may not care
about building the regular .sh installer.  That can be disabled by setting
DARWIN_INSTALLER_SH to "0".  We can specify the prefix path to the SDK with
the "DARWIN_INSTALLER_PATH".  This applies to both in the .dmg and the .pkg.
This path will be relative to the root directory in the .dmg; and relative to
the user's installed location in the .pkg.  For example, with the above
default path, opening the .dmg file through the Finder (or from the terminal
with `open`) will put the sdk files at
"/Volumes/oecore-meta-toolchain-aarch64-armv8a-xxx-xxx/Library/Developer/org.openembedded.sdk/oecore-meta-toolchain-aarch64-armv8a-xxx-xxx/".
For the .pkg, the user will select the Volume or home directory, then the
"/library/Developer/org.openembedded.sdk/..." path will be relative to that.
If the user picks "Install for me only" option, it will be in their home
directory (as ~/Library/Developer/...).

Open questions:

1. Both the .dmg and .pkg installer have the ability to attach license
  documents - should we put anything in there?  I suppose this could also have
 some basic instructions for the pathing and use of the SDK.  ` 2. The .pkg has
  a few screens and text instructions that can be displayed while running the
  installer.  Should we put anything in there?

3. The .dmg is intentionally set up as an uncompressed image.  This is
  intended to speed up compiles for the target.  Requiring the os to constantly
  decompress data seems like a waste of time.  However, it is possible that the
  macOS cache will handle it properly and it won't be an issue.  Should we have
  a local.conf option to enable compression?  Alternatively, we could compress
  the generated .dmg with `xz` like the .sh installer does.

4. Maybe the most controversial question is what should the default
  DARWIN_INSTALLER_PATH be?  Or should it be DARWIN_INSTALLER_PREFIX, since it's
  more of a prefix than a full path?  This is probably depends on whether we
  want the use to be "more macOS" or more "Linux / Yocto".  More macOS would
  expect the users to have multiple .dmg's mounted for individual SDKs they
  intend to use.  Linux / Yocto would have them separated at the lower installed
  level - as /usr/local/${SDK_NAME_PREFIX}-${SDK_ARCH}", with different SDKs
  having different SDK_NAME_PREFIXes.  Should DARWIN_INSTALLER_PATH be replaced
  with SDKPATHINSTALL?

5. What does the post-install script really need to do to relocate things on
  macOS?  I've got to admit that I really can't follow all the logic between
  `toolchain-shar-extract.sh`, `toolchain-shar-relocate.sh`, `relocate_sdk.py`,
  and the post-relocate-setup.d/* scripts.  It looks like most of the complexity
  is to deal with linux' ld.so pathing - which isn't relevant on macOS.  The
  finalize SDK scripts I have here do two things - run `sed` on text files to
  change the installed paths; and run codesign to sign the binaries.  Is there
  more we need to do?

Eric L. Hernes (1):
  Add DMG and PKG SDK installers

 README                                      |  19 ++
 conf/files/darwin-toolchain-shar-extract.sh | 316 ++++++++++++++++++++
 conf/files/finalize-sdk.sh                  |  48 +++
 conf/files/mk-macos-dmg.sh                  |  49 +++
 conf/files/mk-macos-pkg.sh                  | 161 ++++++++++
 conf/files/post-install.in                  |   9 +
 conf/files/pre-install.in                   |  84 ++++++
 conf/layer.conf                             |   3 +
 conf/machine-sdk/darwin-common.inc          |  54 +++-
 9 files changed, 741 insertions(+), 2 deletions(-)
 create mode 100644 conf/files/darwin-toolchain-shar-extract.sh
 create mode 100644 conf/files/finalize-sdk.sh
 create mode 100755 conf/files/mk-macos-dmg.sh
 create mode 100755 conf/files/mk-macos-pkg.sh
 create mode 100644 conf/files/post-install.in
 create mode 100644 conf/files/pre-install.in