Expand currently unused flash space to roofs for DIR-825-B1 by using the same
flash space as the old ar71xx big image without moving the caldata.
With some testing this partition is use by the OEM firmware
but if changed is regenerated which allows reverting to OEM firmware
Signed-off-by: Alan Luck <luckyhome2008@gmail.com>
(cherry picked from commit aca8bb5cc332f0ffdf4249e76b0a56716f98bef0)