ramips: mt76x8: add support for Keenetic Extra (KN-1713)

ramips: mt76x8: add support for Keenetic Extra (KN-1713)

Specification:
SoC: MediaTek MT7628AN
RAM: 128M DDR2, Etron Technology EM68C16CWQG-25H
Flash: 32M, cFeon EN25QH256A (Dual Boot, SPI)
Switch: MediaTek MT7628AN, 4 ports 100 Mbps
WiFi: MediaTek MT7628AN 2.4 GHz 802.11n and MediaTek MT7613BEN 5 GHz 802.11ac
USB: 1 port USB 2.0
GPIO: 3 buttons (Wi-Fi, Reset, FN), 4 LEDs (Power, Internet, FN, Wi-Fi), USB port power controls

Disassembly:
There are 2 screws at the bottom. After removing the screws, pry the gray plastic part around (it is secured with latches) and remove it.

UART Interface:
The UART interface can be connected to the 5 pin located between the WAN port and the RESET button.
Pins (from WAN port to Reset button): VCC, TX, RX, NC, GRD
Settings: 115200, 8N1

Flashing via OEM recovery software:
1. Download the OEM recovery software from the manufacturer's website
2. Download the firmware image (for OpenWRT it is *-squashfs-factory.bin), rename it to KN-1713_recovery.bin
3. Replace the file in the fw folder OEM recovery software with the file from step 2.
4. Run the OEM recovery software and follow the instructions.

Flashing via TFTP:
1. Connect your PC and router to port 1-3, configure PC interface using IP 192.168.1.2, mask 255.255.255.252
2. Serve the firmware image (for OpenWRT it is *-squashfs-factory.bin) renamed to KN-1713_recovery.bin via TFTP
3. Power up the router while pressing Reset button on the back
4. Release Restart button when Power LED starts blinking

To revert back to OEM firmware:
The return to the OEM firmware is carried out by using the methods described above with the help of the appropriate firmware image.

When using OEM bootloader, the firmware image size cannot exceed the size of one OEM «Firmware_x» partition or Kernel + rootFS size.

Signed-off-by: Anton Yu. Ivanusev <ivanusevanton@yandex.ru>
Link: https://github.com/openwrt/openwrt/pull/17382
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>