Sysext images have a compatibility matching mechanism that searches for
the matching OS version or custom sysext level setting. On Flatcar
there is just the OS version set in /etc/os-release until now which
means that sysext images can't easily be used together with autoupdates
that change the OS version.
Define a sysext level for Flatcar so that users can refer to it instead
of the OS version when they have images that don't rely on a particular
Flatcar version.
Here an example of the now possible metadata:
/etc/extensions/NAME/usr/lib64/extension-release.d/extension-release.NAME
ID=flatcar
SYSEXT_LEVEL=1.0
and a symlink /etc/extensions/NAME/usr/lib → /etc/extensions/NAME/usr/lib64
to work around the problem that using lib/ as path destroys Flatcar's
lib → lib64 symlink.
In the future the matching logic hopefully gets more flexible because
now it is just a string comparison. Also, the architecture is not
matched either for now - we should work with upstream to improve this.
Closes: https://github.com/flatcar-linux/Flatcar/issues/643
