postmarketos-ondev: support optional SD to eMMC installation (0.4.0)

Summary

• Add new deviceinfo variables and use them for pine64-pinephone and qemu-amd64 (see reference):
  • dev_internal_storage
  • dev_internal_storage_repartition
• postmarketos-mkinitfs: prefer to use a partition with label pmOS_inst_boot over pmOS_boot. This is needed to make sure the installer OS knows the partition it needs to boot from, even if postmarketOS boot partition is present twice (on SD from installer OS and on eMMC from installed OS). It's inst and not install because of a character limit.
• postmarketos-base: retire devmappings service. This one is obsolete and breaks the logic in postmarketos-mkinitfs.
• calamares-extensions: upgrade to 1.1.0
• postmarketos-ondev: upgrade to 0.4.0

Find details in the commit messages.

How to test and photos

Will be posted shortly in postmarketos-ondev#22 (closed).