ath79: add support for I-O DATA WN-AC1600DGR2

I-O DATA WN-AC1600DGR2 is a 2.4/5 GHz band 11ac router, based on
Qualcomm Atheros QCA9557.

Specification:

- Qualcomm Atheros QCA9557
- 128 MB of RAM
- 16 MB of Flash
- 2.4/5 GHz wifi
  - 2.4 GHz: 2T2R (SoC internal)
  - 5 GHz: 3T3R (QCA9880)
- 5x 10/100/1000 Mbps Ethernet
- 6x LEDs, 6x keys (4x buttons, 1x slide switch)
- UART header on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - 115200n8

Flash instruction using factory image:

1. Connect the computer to the LAN port of WN-AC1600DGR2
2. Connect power cable to WN-AC1600DGR2 and turn on it
3. Access to "http://192.168.0.1/" and open firmware update page
("ファームウェア")
4. Select the OpenWrt factory image and click update ("更新") button
5. Wait ~150 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>

include/image-commands.mk

@@ -264,6 +264,11 @@ define Build/openmesh-image
 		"$(call param_get_default,rootfs,$(1),$@)" "rootfs"
 endef
 
+define Build/senao-header
+	$(STAGING_DIR_HOST)/bin/mksenaofw $(1) -e $@ -o $@.new
+	mv $@.new $@
+endef
+
 define Build/sysupgrade-tar
 	sh $(TOPDIR)/scripts/sysupgrade-tar.sh \
 		--board $(if $(BOARD_NAME),$(BOARD_NAME),$(DEVICE_NAME)) \