* use binary provided by MediaTek to work-around 'bromimage' issue
* use @OPENWRT mirror for blobs
* refactor Makefile
* add mt7622 1c variants (using binaries provided by MTK)
(cherry picked from commit 068c82039f and
commit 9cd089dbbf)
Signed-off-by: Daniel Golle <daniel@makrotopia.org>