ath79: add support for Arduino Yun

Arduino Yun is a microcontroller development board, based on Atmel
ATmega32u4 and Atheros AR9331.

Specifications:
- MCU: ATmega32U4
- SoC: AR9331
- RAM: DDR2 64MB
- Flash: SPI NOR 16MB
- WiFi:
  - 2.4GHz: SoC internal
- Ethernet: 1x 10/100Mbps
- USB: 1x 2.0
- MicroSD: 1x SDHC

Notes:
- Stock firmware is based on OpenWrt AA.
- The SoC UART can be accessed only through the MCU.
  YunSerialTerminal is recommended for access to serial console.
- Stock firmware uses non-standard 250000 baudrate by default.
- The MCU can be reprogrammed from the SoC with avrdude linuxgpio.

Installation:
1.  Update U-Boot environment variables to adapt to new partition scheme.
    > setenv bootcmd "run addboard; run addtty; run addparts; run addrootfs; bootm 0x9f050000 || bootm 0x9fea0000"
    > setenv mtdparts "spi0.0:256k(u-boot)ro,64k(u-boot-env),15936k(firmware),64k(nvram),64k(art)ro"
    > saveenv
2.  Boot into stock firmware normally and perform sysupgrade with
    sysupgrade image.
    # sysupgrade -n -F /tmp/sysupgrade.bin

Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ath79/image/generic.mk

@@ -210,6 +210,17 @@ define Device/alfa-network_ap121f
 endef
 TARGET_DEVICES += alfa-network_ap121f
 
+define Device/arduino_yun
+  SOC := ar9331
+  DEVICE_VENDOR := Arduino
+  DEVICE_MODEL := Yun
+  DEVICE_PACKAGES := kmod-usb2 kmod-usb-chipidea2 kmod-usb-ledtrig-usbport \
+	kmod-usb-storage block-mount -swconfig
+  IMAGE_SIZE := 15936k
+  SUPPORTED_DEVICES += arduino-yun
+endef
+TARGET_DEVICES += arduino_yun
+
 define Device/aruba_ap-105
   SOC := ar7161
   DEVICE_VENDOR := Aruba