libfstools: partname: raise priority to 25

libfstools: partname: raise priority to 25

Make sure partname has precedence over UBI and MTD.
We already got ways to prevent partname for matching partitions if
needed, however, we would need them as well for UBI and MTD to prevent
use of UBI rootfs_data despite having started from MMC on boards
having both (such as the BPi-R3).

Reported-by: Rui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>