Add support for initial-setup

I kept that minimal for now but I think this should be enough so that we can skip the initial use (and thus have home on separate, encrypted) partitions which should help !11 (merged) . This needs #2 (closed) fixed before it can land and I need to add g-i-s to the nightly builds as we need two small patches.

TODO

• Drop fixed phosh user
• Add system-homed support once !12 (merged) lands