Remove systemd user session and clean up packagegroups and images

Remove systemd user session and clean up packagegroups and images

Remove the use of systemd user session to align with how upstream
runs Weston, and to allow using all systemd sandboxing features with
the compositor and homescreen, launcher, etc. applications.  The
changes for this touched enough packagegroups and images that further
rework was done to address some of the cleanup described in SPEC-4813,
see below for details.

Changes:
- Rework systemd units and recipes of affected applications.
- Rework images to pull in image .bb files instead of .inc files.
- Pull in the new agl-image-compositor as a base image where
  appropriate.
- Update weston-terminal-conf recipe to use new split out
  weston-terminal package.
- Use new agl-crosssdk bbclass where appropriate.
- Pull some local common cross-SDK additions into a new
  agl-ivi-crosssdk.inc include file and use as appropriate.  Note
  that this does have a side effect of likely correcting the contents
  of agl-image-ivi-crosssdk for some recent additions.
- Remove flutter-gallery bbappend that turned it into an app, as we
  are not using it in images, have other examples now, and it seems
  better to not affect the contents of agl-image-flutter.
- Remove unused agl-image-graphical-html5. If a downstream user does
  desire an image with Chromium+WAM without applications, restoring it
  can be considered.
- Remove unused agl-image-graphical-qt5-crosssdk.
- Remove all packagegroup-agl-profile-* packagegroups, as they were
  either empty or not providing value at this point.  Any future plans
  for defining profiles can start from a clean sheet.
- Added new packagegroup-agl-ivi-multimedia-hardware to hold some
  rcar3 specific additions (via override) that were previously being
  appended onto packagegroup-agl-graphical-multimedia even though
  they are machine-specific and that packagegroup is not.  This may
  be a stopgap solution if the agl-ivi-* packagegroups are reworked
  further.
- Replace some :append usage with += to avoid creating problems
  for downstream users.

Bug-AGL: SPEC-4714, SPEC-4813

Change-Id: I544b1495bed1e2e2412a8e46b7d20d7622ec28c9
Signed-off-by: Scott Murray <scott.murray@konsulko.com>