classes: add a systemd-sysext image classabelloni/master-next-success

systemd-sysext can load a raw-image containing usr/ and opt/ folders
to mount them as RO overlay over the rootfs, to "extend" the systems.

This class provides the necessary changes/additions to the enclosed
file-system so that systemd-sysext accepts the extension for "merge"
into the rootfs.

With such an created image, placed into the correct folder (see [1]),
`systemd-sysext list` should be able to list the "extension" and
`systemd-sysext merge` should enable the overlay. On both commands a
preceding "SYSTEMD_LOG_LEVEL=debug" can aide in figuring out what is
amiss.

The strict name checking systemd-sysext does against the name of
extension-release.NAME file, is disabled, as there is only one such in
the resulting image. This is done to allow a user to freely rename the
resulting image file.
Note that for e.g. squashfs, the kernel needs CONFIG_SQUASHFS_XATTR=y

Link: https://www.freedesktop.org/software/systemd/man/latest/systemd-sysext.html
Link: https://0pointer.net/blog/testing-my-system-code-in-usr-without-modifying-usr.html
Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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