Draft: main/mkinitfs: add patch to mount /usr from /etc/fstab if exists

!71694 (merged) could have the unintended consequence of people using /usr in a different partition to end up with non-bootable systems (if they expect everything to be under /bin, /sbin, and /lib). This is quite unlikely, but theoretically still possible. So leaving it here if we think it's worth going over it (after it was merged in mkinitfs!178 (merged)). I would have had a mkinitfs release myself if I had the ability to