[PATCH v6 0/3] pkg-database and systemd-sysext image

[Johannes Schneider <johannes.schneider@leica-geosystems.com>]

systemd-sysext allows to overlay another image (or multiple) ontop of
a "base-image" = the current rootfs, via the use of overlayfs; to add
tools and features meant for development purposes.

To quote the documentation on systemd-sysext:
" ...addition in order to make debugging/development easier). System
extension images should not be misunderstood as a generic software
packaging framework, ..."

To build a lean image, that only holds packages that are not already
part of the base-image, a snapshot of the package-database is taken
after the installation of the base-rootfs is done, and picked up again
when collecting the rootfs of such a extension image.

with all this in place an example usage could look like this:
some-core-image.bb
  inherit core-image
  IMAGE_GEN_PKGDBFS = "1"

extending-image.bb
  inherit image-sysext
  IMAGE_FSTYPES = "squashfs"
  IMAGE_BASE_PKGDB = "some-core-image"
  # the above pointing at a package-db similar to:
  # build/deploy/images/$MACHINE/some-core-image-$MACHINE-20240210172305-pkgdb.rootfs.tar.gz

then on the device, running some-core-image, with the extension image placed at FN:
$> ln -s "$FN" /run/extensions/$(basename $FN).raw
$> systemd-sysext list
$> SYSTEMD_LOG_LEVEL=debug systemd-sysext merge

As long as the VERSION_ID of the extension image matches the os-release
in the base image, the above commands return sucessfully;
for details on the compativility check see the docs for systemd-sysext.

=========

changes with v2:
        rebase from 'kirkstone' onto 'master'

changes with v3:
	incorporate review suggestions for simplification
	add task dependency handling
	add oe-selftest for the pkgdb handling
	add variable documentation and
	some more comments, and examples in the commit-msg

changes with v4:
	rebase onto 'master' => no functional changes
	fixed patchtest findings

changes with v5:
        rebase onto 'master'
	add '.sysext' to the deployed symlink name
        sidenote on the tests and autobuilder failure: run locally they succeed, e.g.:
          #> oe-selftest --verbose -r imagefeatures.ImageFeatures.test_image_gen_pkgdbfs

changes with v6:
	restructure test to call 'bitbake' only once in the testcase itself
	  (in hopes of solving the autobuilder problem; local test runs succeed)