meta-agl-core: Update weston/agl-compositor init

meta-agl-core: Update weston/agl-compositor init

Changes/rework to get weston and agl-compositor starting again:
- Since an upgrade to a newer Yocto release is in the near future,
  update weston-init and associated files to effectively backport
  the new weston startup behavior added in 3.3/Hardknott as our
  new base.  The changes mean weston or agl-compositor will by
  default start as a "weston" user that replaces the "display"
  user that had been added previously in AGL.  The goal is that
  any new work done on top of this base should hopefully work
  on 3.5/Kirkstone without further substantial rework.
- Add new agl-compositor-init recipe that replaces the previous
  weston-init bbappend in meta-agl-demo.  Having it as a separate
  recipe in core so weston or agl-compositor "just work" in simple
  test images seems like a better approach.
- As part of the above, drop the --log option to agl-compositor
  in its command-line to address SPEC-4112.
- Add SYSTEMD_DEFAULT_TARGET definition to agl-image-weston and
  in a new core-image-weston bbappend to result in agl-compositor
  and weston starting automatically in the corresponding images.
  This is required with the new weston-init behavior until we
  upgrade past 3.3/Hardknott, when "weston" in IMAGE_FEATURES
  can be used instead.

Bug-AGL: SPEC-4121, SPEC-4112

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ia64894416846569abf8e744006ef26637279a895
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26782
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>