Build support for distro-independent sysexts in SDK based on Gentoo prefix

[pothos]

Current situation

Systemd-sysext images that are not coupled to the OS version need to contain their dependencies and show not assume something about /usr/lib nor ship libraries in that path. They have to use /usr/local/ or /opt/ as prefix.

Impact

This requires manual installation with ./configure --prefix= which is a bit hard to use.

Ideal future situation

Gentoo packages could be used through the prefix option that allows to install everything to a custom path. A build helper in the SDK can use this to generate version-uncoupled (even distro independent) systemd-sysext images.

Implementation options

Additional information

[t-lo]

Will tackle this in the Hackathon next week: https://hackbox.microsoft.com/hackathons/hackathon2023/project/30944

[pothos]

Can we close this as it's done https://github.com/flatcar/scripts/blob/main/PREFIX.md even if experimental?