ramips: add support for D-Link COVR-X1860 A1

ramips: add support for D-Link COVR-X1860 A1

The COVR-X1860 are MT7621-based AX1800 devices (similar to DAP-X1860, but
with two Ethernet ports and external power supply) that are sold in sets
of two (COVR-X1862) and three (COVR-X1863).

Specification:
 - MT7621
 - MT7915 + MT7975 2x2 802.11ax (DBDC)
 - 256MB RAM
 - 128 MB flash
 - 3 LEDs (red, orange, white), routed to one indicator in the top of the device
 - 2 buttons (WPS in the back and Reset at the bottom of the device)

MAC addresses:
 - LAN MAC (printed on the device) is stored in config2 partition as ASCII (entry factory_mac=xx:xx:xx:xx:xx:xx)
 - WAN MAC: LAN MAC + 3
 - 2.4G MAC: LAN MAC + 1
 - 5G MAC: LAN MAC + 2

The pins for the serial console are already labeled on the board (VCC, TX, RX, GND). Serial settings: 3.3V, 115200,8n1

Flashing via OEM Web Interface:
 - Download openwrt-ramips-mt7621-dlink_covr-x1860-a1-squashfs-factory.bin via the OEM web interface firmware update
 - The configuration wizard can be skipped by directly going to http://192.168.0.1/UpdateFirmware_Simple.html

Flashing via Recovery Web Interface:
 - Set your IP address to 192.168.0.10, subnetmask 255.255.255.0
 - Press the reset button while powering on the deivce
 - Keep the reset button pressed until the status LED blinks red
 - Open a Chromium based browser and goto http://192.168.0.1
 - Download openwrt-ramips-mt7621-dlink_covr-x1860-a1-squashfs-recovery.bin

Revert back to stock using the Recovery Web Interface:
 - Set your IP address to 192.168.0.10, subnetmask 255.255.255.25
 - Press the reset button while powering on the deivce
 - Keep the reset button pressed until the status LED blinks red
 - Open a Chromium based browser and goto http://192.168.0.1
 - Flash a decrypted firmware image from D-Link. Decrypting an firmware image is described below.

Decrypting a D-Link firmware image:
 - Download https://github.com/openwrt/firmware-utils/blob/master/src/dlink-sge-image.c and https://raw.githubusercontent.com/openwrt/firmware-utils/master/src/dlink-sge-image.h
 - Compile a binary from the downloaded file, e.g. gcc dlink-sge-image.c -lcrypto -o dlink-sge-image
 - Run ./dlink-sge-image COVR-X1860 <OriginalFirmware> <OutputFile> -d
 - Example for firmware 102b01: ./dlink-sge-image COVR-X1860 COVR-X1860_RevA_Firmware_102b01.bin COVR-X1860_RevA_Firmware_102b01_Decrypted.bin -d

The pull request is based on the discussion in https://forum.openwrt.org/t/add-support-for-d-link-covr-x1860

Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>
Signed-off-by: Roland Reinl <reinlroland+github@gmail.com>