ramips: mt7621: ASUS RT-AX53U add NMBM, nest firmware

ramips: mt7621: ASUS RT-AX53U add NMBM, nest firmware

Nests kernel and ubi into firmware partition in-order to be compatible
with OEM firmware. This allows restoring oem firmware from a backup of
firmware2. Add jffs2 partition which is present in the oem firmware.
Add support for mediatek NMBM (wear leveling on newer mediatek devices).
Exclude UBI partition from NMBM management.
Continues PR #10685.

Tested-by: Felix Baumann <felix.bau@gmx.de>
Signed-off-by: Felix Baumann <felix.bau@gmx.de>
(cherry picked from commit 787ecdf66de6798540fb8b756404566341b4733c)