postmarketos-initramfs: unify with minimal initramfs (MR 5636)

We previously forked the initramfs into a minimal variant for highly
space constrained devices, to allow the normal variant to gain
additional functionality like the debug-shell (With OSK). Initially the
hope was that only a handful of devices would need the minimal variant,
but in practise this has not been the case.

Trying to maintain two diverging forks of the initramfs is absoutely
unsustainable and (to be frank) a bit silly.

Based on discussion with craftyguy and newbyte, let's re-architect the
initramfs to run in two stages, with the second stage being part of
initramfs-extra. By itself this doesn't make a lot of sense, since
requiring the boot partition be mounted for the debug-shell to be useful
is a major downgrade.

However this can be used in tandem with pending support for a new
deviceinfo_create_initfs_extra variable, this is default false in
mkinitfs which finally gets rid of initramfs-extra entirely for devices
that don't need it.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Co-authored-by: Clayton Craft <clayton@craftyguy.net>