Add basic U-Boot drop-in replacement compatible with the flash layout
of the vendor loader of the Zbtlink WG3526 (16M) MT7621 router board.
The idea here is a to have a reference build of uboot-mediatek also for
a simple MIPS boards more popular than MT7621 RFB.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
