ramips: add support for TP-Link RE200 v1

ramips: add support for TP-Link RE200 v1

TP-Link RE200 v1 is a wireless range extender with Ethernet and 2.4G and 5G
WiFi with internal antennas. It's based on MediaTek MT7620A+MT7610EN.

Specifications
--------------

- MediaTek MT7620A (580 Mhz)
- 64 MB of RAM
- 8 MB of FLASH
- 2T2R 2.4 GHz and 1T1R 5 GHz
- 1x 10/100 Mbps Ethernet
- UART header on PCB (57600 8n1)
- 8x LED (GPIO-controlled; only 6 supported), 2x button

There are 2.4G and 5G LEDs in red and green which are controlled
separately. The 5G LED is currently not supported, since the GPIOs couldn't
be determined.

Installation
------------

Web Interface
-------------

It is possible to upgrade to OpenWrt via the web interface. However, the
OEM firmware upgrade file is required and a tool to fix the MD5 sum of
the header. This procedure overwrites U-Boot and there is not failsafe /
recovery mode present! To prepare an image, you need to take the header
and U-Boot (i.e. 0x200 + 0x20000 bytes) from an OEM firmware file and
attach the factory image to it. Then fix the header MD5Sum1.

Serial console
--------------

Opening the case is quite hard, since it is welded together. Rename the
OpenWrt factory image to "test.bin", then plug in the device and quickly
press "2" to enter flash mode (no line feed). Follow the prompts until
OpenWrt is installed.

Unfortunately, this devices does not offer a recovery mode or a tftp
installation method. If the web interface upgrade fails, you have to open
your device and attach serial console. Since the web upgrade overwrites
the boot loader, you might also brick your device.

Additional notes
----------------

MAC address assignment is based on stock-firmware. For me, the device
assigns the MAC on the label to Ethernet and the 2.4G WiFi, while the 5G
WiFi has a separate MAC with +2.

*:88    Ethernet/2.4G    label, uboot 0x1fc00, userconfig 0x0158
*:89    unused           userconfig 0x0160
*:8A    5G               not present in flash

This seems to be the first ramips device with a TP-Link v1 header. The
original firmware has the string "EU" embedded, there might be some region-
checking going on during the firmware upgrade process. The original
firmware also contains U-Boot and thus overwrites the boot loader during
upgrade.
In order to flash back to stock, the first header and U-Boot need to be
stripped from the original firmware.

Signed-off-by: Andreas Böhler <dev@aboehler.at>