ramips: mt7621: add support for Wodesys WD-R1802U

ramips: mt7621: add support for Wodesys WD-R1802U

This commit adds support for a dual-band AX1800 wall plug manufactured
by Shenzhen Century Xinyang Tech Co., Ltd.

CPU:	Mediatek MT7621A (2 cores, 4 threads)
RAM:	256i MiB DDR3 (Samsung K4B2G1646F-BCNB)
ROM:	16 MiB SPI NOR (Winbond W25Q128JVPQ)
Wired:	one gigabit RJ45 port (with green/yellow non-GPIO LEDs)
WiFi:	Mediatek MT7905DAN + MT7975DN (DBDC 2x 2T2R)
Ant.:	four 2 dBi external antennas (two 2.4GHz, two 5 GHz)
GPIO:	tri-color status LED (GPIO 13, 14, 16);
	reset button (GPIO 18)
Power:	12V 2-pin JST-XH on main PCB
	110/220V AC to 12V1A DC on auxiliary PCB
UART:	115200 8n1, SMD pads available on the PCB as J4
	pinout is [3v3] (Rx) (Tx) (Gnd)
MAC:	1C:BF:CE:xx:xx:xx	(2.4 GHz, label)
	1C:BF:CE:xx:xx:xx + 1	(ethernet [1])
	1C:BF:CE:xx:xx:xx + 2	(5 GHz)

Original firmware is LEDE Reboot 17.01-SNAPSHOT (kernel 4.4.198)
with a few custom packages and a non-LuCI web interface.
Telnet and SSH are enabled, requiring an unknown root password [2].
Root password is also needed to access the router via UART console,
but passwordless telnet can be enabled via a trivial web exploit [3]
and then the root password can be removed by editing `/etc/shadow`.

Installation: First upload `sysupgrade` binary via web interface at
  `http://192.168.188.1/settings.shtml` and wait until getting back to
  the home screen (select network to extend). The installation fails
  since the original firmware uses `swconfig` and recent versions of
  OpenWrt use DSA. However, the sysupgrade file is uploaded correctly
  and stored at `/tmp/upgrade.bin`, so it can be written to flash via
  the web exploit [4] (both `mtd -r write` and `sysupgrade -Fn` work
  fine). Passwordless telnet/ssh is not needed for installation.
  Alternatively, use u-boot menu to load image via TFTP.

Notes:
- Device model in LEDE is "MediaTek MT7621 RFB (802.11ax,SNOR)".
- It is sold under several names, among them are Wodesys WD-R1802U,
  Fenvi F-AX1802U, and EDUP EP-2971; the Wodesys brand was selected
  since it is referenced in `/etc/banner` and `/etc/hosts`, and the
  PCB is marked "WD518A V1.0".
- Instead of a standard ethernet transformer, the PCB has a few tiny
  SMD coils.

[1] Original firmware sets ethernet MAC to 1C:BF:CE:E7:62:1D based on
    offset `0x3fff4` in the Factory partition; since this is the same
    MAC for all units, whereas WiFi MACs stored at offsets 0x6 and 0xc
    are unique, it was decided to use <label MAC + 1> for ethernet.
[2] root:$1$7rmMiPJj$91iv9LWhfkZE/t7aCBdo.0:18388:0:99999:7:::
[3] curl -X POST http://192.168.188.1/cgi-bin/adm.cgi \
    -d page=Lang -d langType="en;killall telnetd;telnetd -l /bin/sh"
[4] curl -X POST http://192.168.188.1/cgi-bin/adm.cgi \
    -d page=Lang -d langType="en;mtd -r write /tmp/upgrade.bin firmware"

Signed-off-by: Rani Hod <rani.hod@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15777
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>