realtek: EnGenius EWS2910P: use the mtd3 partition for root overlay

realtek: EnGenius EWS2910P: use the mtd3 partition for root overlay

The root overlay is mounted on the "rootfs_data" partition. This comes
at the end of the firmware image, courtesy of mtdsplit. There is very
little space left (About 1MB), which can fill up rapidly.

The "firmware" and "firmware2" partitions are part of the bootloader
dual firmware logic. They should contain independent, valid uImages.
This leaves "jffs2-cfg" (mtd3) and "jffs2-log" (mtd4) as candidates.

mtd3 is about 13.7 MB and is used by the vendor firmware to store
configuration settings. It is only erased by vendor firmware during a
factory reset. By naming this partition "rootfs_data", it becomes the
root overlay, providing significantly more room. Even with mtdsplit
wanting to create a "rootfs_data" on the firmware partition, mtd3 is
used as the overlay.

Rename "jffs2-cfg" to "rootfs_data", and profit from the extra space.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>