mediatek: mt7623: prepare for full sysupgrade support on UniElec U7623

mediatek: mt7623: prepare for full sysupgrade support on UniElec U7623

An upcoming commit will add a full system image for U7623 which will
contain the MBR partition table and U-Boot too.

That contrasts with the current image which only owns the eMMC from
sector 0xa00 onwards, and must start with a legacy uImage.

Prepare for sysupgrade to the new images, and cope with the fact that
the recovery partition will be /dev/mmcblk0p2 instead of /dev/mmcblk0p1
after the upgrade.

This commit could potentially be backported to 19.07 to allow for direct
sysupgrade to the new image layout.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>