mediatek: set new compat version if booted on R64 and R3

mediatek: set new compat version if booted on R64 and R3

If the board comes up with OpenWrt that means that the bootloader is
recent enough and knows about the new device tree overlays.

Using /etc/board.d/ is not enough in this case because it doesn't
overwrite existing configuration which may exist (and is fine to exist)
if the user updated with 'sysupgrade -F *.itb' and has kept
configuration. They would still need to manually set compat_version
even though the fact that the bootloader env has been updated can be
implied by the fact that the system has started.

Hence we can always set compat_version=1.1 for those two boards using
uci-defaults.

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