pmb.install: add support for creating btrfs root filesystem

In preparation of reversible upgrades, add btrfs support for the creation of the root filesystem.

Note that postmarketos-mkinitfs currently does not support booting from btrfs, which means this MR depends on an MR in postmarket-mkinitfs which is still pending.

Depends on pmaports Draft: main/postmarketos-mkinitfs: basic support for btrfs

Recursively depends on postmarketos-mkinitfs main: Add btrfs from btrfs-progs to initramfs-extra