Right now we fallback to the old behavior of searching for the "pmOS_root"
label when the root partition specified by pmos_root_uuid= on the cmdline
is not found. This works fine most of the time, but there are edge cases in
which this does not behave correctly. Problems occur especially if there
are multiple installations of pmOS (e.g. one on internal storage, and one
on USB drive/SD card). Depending on timing or enumeration order, the system
might boot into the wrong rootfs if the intended root partition shows up
too late.

For the boot partition we already enforce the UUID provided on the cmdline.
Let's apply the same for the root partition to ensure we always boot into
the correct root partition.