main/postmarketos-initramfs: mount subpartitions if root or boot is missing

Commit 7030e08e ("postmarketos-initramfs: check for root subpartition (MR 5048)") changed the partition check in mount_subpartitions to look for the root partition instead of the boot partition, in order to mount subpartitions even if the pmOS_boot partition was already found.

Unfortunately, this causes regressions for the opposite case: If we find a root partition, but the boot partition exists only as a subpartition then mount_subpartitions will return early, without actually making the boot partition accessible.

This is easy to trigger if you have a different Linux distribution with an encrypted crypto_LUKS volume installed and then flash pmOS subpartitions to a single partition. find_root_partition() will return the crypto_LUKS partition (without checking if it belongs to pmOS) and the boot partition is not found.

Fix both use cases by checking for root and boot partition. If any of them is not found, we try to mount the subpartitions.