image: add FACTORY_SIZE to DEFAULT_DEVICE_VARS

image: add FACTORY_SIZE to DEFAULT_DEVICE_VARS

FACTORY_SIZE is used as a device recipe variable on both the D-Link
DIR-825-B1 and Trendnet TEW-673GRU, but is not listed as a device
variable, neither globally, nor for ath79. Being assigned the same
value, this probably hasn't caused any issues.

Add FACTORY_SIZE to the global list DEFAULT_DEVICE_VARS, to ensure the
variable is reset for every device, and to allow it to be used outside
of the ath79 target.

Signed-off-by: Sander Vanheule <sander@svanheule.net>