mediatek: rework Bananapi BPi-R2

mediatek: rework Bananapi BPi-R2

 * introduce recovery (=initramfs) vs. production dual boot scheme
 * make use of uImage.FIT (instead of FAT partition)
 * generate images using build steps (instead of external scripts)
 * simplify sysupgrade and config restore (thanks to uImage.FIT)
 * make sure mmc devices are ordered persistently (set DT aliases)

This commit breaks sysupgrade from existing installations, you will
have to re-install using the sdcard.img.gz image.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>