ath79: make Engenius fakeroot partitions read-only

ath79: make Engenius fakeroot partitions read-only

For:

 - ENH202 v1
 - ENS202EXT v1

These boards were committed before it was discovered
that for all Engenius boards with a "failsafe" image,
forcing the failsafe image to load next boot
can be achieved by editing the u-boot environment like:

  `fw_setenv rootfs_checksum 0`

So it's not necessary to delete a partition to boot to failsafe image.

Signed-off-by: Michael Pratt <[email protected]>