* 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 068c82039f5192a79e2139db42fdc734702da5a3 and
commit 9cd089dbbfe07b61590dd214957bc21bfdc7fd5d)
Signed-off-by: Daniel Golle <daniel@makrotopia.org>