config: fixes + handle mirrors migration (MR 2252)

Get rid of config.mirror_alpine and mirrors_postmarketos and make sure
they get migrated over for existing users.

mirrors_postmarketos being a list was always a bit off, but now we have
per-aports mirrors which make a lot more sense for what we're trying to
do with systemd.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>