Add meta-agl-lxc

Add meta-agl-lxc layer with LXC container integration recipes, and
"agl-lxc" feature to enable its use.  The changes are based on the
previous prototypes by Tadao Tanikawa and the updates done for the
trial integration layers on the agl-ic-eg github by Naoto Yamaguchi,
Hiroyuki Ishii, and Jan-Simon Moeller.

The layer can be used by running aglsetup.sh with just the "agl-lxc"
feature, then building lxc-host-image-demo with bitbake.

Notes:
- The agl-lxc feature enables a "agl-container-guest" multiconfig
  configuration for building the guest image(s).  This suffices for
  demonstrating integrated builds with multiconfig, but may change
  in the future with extension of aglsetup.sh to add more
  flexibility.
- There are guest-image-minimal and guest-image-cluster-demo recipes
  that are intended as a base container template and a demo of its
  use.  The cluster-demo image contains the demo cluster reference
  UI application, and is roughly equivalent to what is named
  cluster-guest in the agl-ic-eg trial integration configuration.
- There are likewise lxc-host-image-minimal and lxc-host-image-demo
  images.  The host demo is configured to pull in the cluster-demo
  guest image, and configuration has been added such that it will
  automatically boot and start the cluster reference UI application.
- Unlike previous work where the guest container image is added to
  the host image via a recipe that packages up the guest image's
  contents, a more direct scheme is used where the guest image is
  unpacked inside the host image rootfs via the use of the
  ROOTFS_POSTPROCESS_COMMAND mechanism.  This avoids some hassles
  around potentially mismatched user and group id lists in the host
  and guest images, with the additional benefit of being faster.
  However, using this scheme is to some degree only necessary due to
  the use of LXC with unpacked guest image hierarchies.  With other
  container tooling that uses e.g. OCI images, a revert to using
  packages may be preferable.
- The reusable logic in the lxc-host-image-minimal recipe makes some
  assumptions wrt naming.  They are:
  - guest image names have the format guest-image-<name>
  - guest images have a corresponding LXC configuration file recipe
    with the name lxc-config-<name>
  The intent is to simplify reuse of the base minimal recipes, but
  this can be reworked if deemed too restrictive.
- packagegroups for e.g. pipewire and weston have purposefully not
  been defined at this time.  It seems better to wait until it is
  more obvious how useful they would be.

Limitations:
- The host image is currently rebuilt every time to work around an
  issue with multiconfig dependencies that is being investigated
  upstream.  This can hopefully be removed in the near future.
- The demo images are currently in meta-agl-lxc for convenience.
  It may make more sense for them to reside in another location to
  ease shifting some things into meta-agl.
- At the moment, only qemux86-64 has been tested.  The changes to
  enable host and guest configuration tweaks for the rcar3 BSP need
  further investigation and discussion on where they should be
  maintained.

Bug-AGL: SPEC-3953, SPEC-3955

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I69d41f1d1c736f05cf3b4997dbda147879e2e25d

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