Draft: A/B Image

This is WIP, please don't merge for now.

I will try to get a system with an empty / (rootfs) partition and setting up everything from /usr:

• Deploy all extra files to /usr
• Migrate to Debian/forky with phosh-nightly/trixie to have systemd v258
• Introduce the new partition layout, mount by dissect with partlabel filtering
• Create a /usr/lib/tmpfiles.d configuration to populate /etc on first boot
• Use gnome-initial-setup for user creation
• Allow either using a single rw rootfs (for development) or AB read-only /usr and generated rw / rootfs
• Add systemd-sysupdate configuration and deploy update artifacts
• Extend UKI with profile entry for Factory Reset

Any feedback about the next steps and the implementation is always welcome!