init: handle ubi/mtd root mounting like all other root types
Assign a Root_Generic magic value for UBI/MTD root and handle the root
mounting in mount_root like all other root types. Besides making the
code more clear this also means that UBI/MTD root can be used together
with an initrd (not that anyone should care).
Also factor parsing of the root name into a helper now that it can
be easily done and will get more complicated with subsequent patches.