ipq806x: EA8500 fix sysupgrade over stock firmware

When running sysupgrade for the first time over the second partition
(that still had stock firmware) the rootfs wasn't flashed as there
wasn't enough space.
This happend because stock also uses UBI, but the volume name wasn't
recognised and wasn't deleted before flashing.

Signed-off-by: Adrian Panella <ianchi74@outlook.com>