ar71xx: set region code of TP-Link TL-WDR3600/4300 to US

There are currently two stock firmwares for the TL-WDR3600/4300, a US and
a universal version. Both allow installing images with US region code, so
we don't need to provide multiple images.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

target/linux/ar71xx/image/tp-link.mk

@@ -116,6 +116,7 @@ $(Device/tplink-8mlzma)
   BOARDNAME := TL-WDR4300
   DEVICE_PROFILE := TLWDR4300
   TPLINK_HWID := 0x43000001
+  IMAGE/factory.bin := append-rootfs | mktplinkfw factory -C US
 endef
 
 define Device/tl-wdr3500-v1
@@ -134,6 +135,7 @@ $(Device/tplink-8mlzma)
   BOARDNAME := TL-WDR4300
   DEVICE_PROFILE := TLWDR4300
   TPLINK_HWID := 0x36000001
+  IMAGE/factory.bin := append-rootfs | mktplinkfw factory -C US
 endef
 
 define Device/tl-wdr4300-v1-il