Support doing user configuration on first boot

We currently have pmbootstrap create the user account during install. This could instead be handled automatically during first boot (eventually with an initial setup step). We should change the default behaviour to install a service that runs on first-boot which creates a default user account, then when we eventually have an initial setup app we can easily transition to it without needing changes to pmbootstrap.

This has the additional benefit of moving this logic out of pmbootstrap where it can be re-used by other tools.