Improve build_sysext SDK tool to have a generic manglefs script

[pothos]

Current situation

The build_sysext script in the SDK can build version-coupled systemd-sysext images from Gentoo packages.
However, not all Gentoo packages would directly work this way, and postprocessing is needed.

Impact

Harder to use

Ideal future situation

Improve build_sysext SDK tool to have a generic manglefs script that moves entries under /etc/systemd/ to /usr/ and creates a helper service unit that creates directories under /etc and /var and sets up symlinks for config files in there, and runs before other units of this sysext will run. Service enablement should also be done with Upholds drop-ins.

Implementation options

Additional information

See sysext-bakery, the mangle scripts in the SDK, and the Azure OEM sysext for reference.