ath79: specify factory.bin recipe for ASUS RP-AC51

Currently factory.bin image recipe of ASUS RP-AC51 is not specified
explicitly and is thus set to the leaked one from the device recipe
right above, i.e. ASUS PL-AC56. Fix it to avoid potential breakage.

Fixes: 416d4483e878 ("ath79: add support for ASUS RP-AC51")
Signed-off-by: Sungbo Eo <[email protected]>

target/linux/ath79/image/generic.mk

@@ -402,6 +402,8 @@ define Device/asus_rp-ac51
   DEVICE_MODEL := RP-AC51
   IMAGE_SIZE := 16000k
   IMAGES += factory.bin
+  IMAGE/factory.bin := append-kernel | pad-to $$$$(BLOCKSIZE) | \
+	append-rootfs | pad-rootfs
   DEVICE_PACKAGES := kmod-ath10k-ct ath10k-firmware-qca9888-ct \
 	-swconfig
 endef