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 <[email protected]>

include/image.mk

@@ -419,7 +419,7 @@ DEFAULT_DEVICE_VARS := \
   DEVICE_FDT_NUM DEVICE_IMG_PREFIX SOC BOARD_NAME UIMAGE_MAGIC UIMAGE_NAME \
   SUPPORTED_DEVICES IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR \
   UBOOT_PATH IMAGE_SIZE \
-  FACTORY_IMG_NAME \
+  FACTORY_IMG_NAME FACTORY_SIZE \
   DEVICE_PACKAGES DEVICE_COMPAT_VERSION DEVICE_COMPAT_MESSAGE \
   DEVICE_VENDOR DEVICE_MODEL DEVICE_VARIANT \
   DEVICE_ALT0_VENDOR DEVICE_ALT0_MODEL DEVICE_ALT0_VARIANT \