Draft: Implement initial support for f2fs

We need to ship f2fs-tools to actually check the filesystem.
Right now it is a no-op to skip running ext4 specific checks unconditionally.

This also reserves some space for privileged users, which is what we manually do with tune2fs for ext4 when installing OTAs.
(see reserve_root)

Depends on: https://gitlab.com/ubports/core/seeds/ubuntu-touch/-/merge_requests/49