Fixes for "Move post-install scripts to openrc subpackages": fajita, cros

Fix some minor (given the size of the MR) fallout from !5688 (merged).

This should fix most of the failed packages on bpo. See commit messages for details.

Fix #3290 (closed)

Fix:

ERROR: unable to select packages:
  postmarketos-cros-keymap (no such package):

(If @craftyguy or @lucaweiss merges this, can you reset the image status back to queued? but anyone else could merge it as well.)