Move postmarketos-base.trigger logic for writing /etc/inittab to devicepkg-dev

This requires !1713 (closed) to be merged first.

postmarketos-base.trigger modifies /etc/inittab to set up a getty when deviceinfo_getty is set. This enables a TTY on serial consoles.

As part of the effort to properly override config files, we shouldn't write to config files from post-install scripts or trigger files anymore.

To fix this, create one MR that does:

• generate the /etc/inittab file in devicepkg_package.sh, if deviceinfo_getty is set
• all device packages with deviceinfo_getty:
  • add replaces="alpine-baselayout" (provides inittab)
  • add pre-upgrade hook that moves the old /etc/inittab to the backup dir, as described here
  • bump pkgrel, the rebuild will use the new devicepkg-dev and generate the /etc/inittab file