WIP: Make filesystem type configurable

Thanks to @Jarrah for working on this feature upstream in calamares-extensions:

https://github.com/calamares/calamares-extensions/pull/8

TODO:

• get MR merged upstream
• new calamares-extensions release
• teach postmarketos-mkinitfs how to resize the root partition if not using ext4 (pmaports!1850 does it for f2fs)
• add support for btrfs to postmarketos-mkinitfs
• let pmbootstrap pass to ondev-prepare.sh, which file systems are supported by the device
  • let's do it with APKBUILD options pmb:kconfigcheck-f2fs, pmb:kconfigcheck-brfs as @craftyguy suggested in https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/2011#note_544562840 and also make pmbootstrap kconfig check use these
• add a config option for calamares-extension to configure which filesystems show up in the dropdown (right now it's always ext4,f2fs,btrfs or disabling the screen entirely)
• adjust ondev-prepare.sh to configure the possible filesystems in calamares-extensions (mobile.conf), based on what pmbootstrap passed
• let ondev-prepare.sh handle cases of just 1 possible filesystem properly (-> use that), and more than 1 (-> display the selection screen)
• test everything

New postmarketos-ondev package should depend on Alpine packages f2fs-tools and btrfs-progs.

Close #39