libfstools: query drivers by priority

libfstools: query drivers by priority

Instead of individual hacks meant to prioritize the storage backend
drivers, register them with an optional priotity. If set, the higher
priority driver should be considered be considered first.

Prioritize UBI and MTD over the bulk of block device drivers
(partname, rootdisk) which allows removing previous hacks having the
same effect.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>