mediatek: mt7622: modernize Linksys E8450 / Belkin RT3200 UBI build

mediatek: mt7622: modernize Linksys E8450 / Belkin RT3200 UBI build

Move fip and factory into UBI static volumes.
Use fitblk instead of partition parser.

 !! RUN INSTALLER FIRST !!
Existing users of previous OpenWrt releases or snapshot builds will
have to **re-run the updated installer** before upgrading to firmware
after this commit.
DO NOT flash or run even just the initramfs image unless you have
run the updated installer which moves the content of the 'factory'
partition into a UBI volume.

tl;dr: DON'T USE YET!

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