ramips: add support for Ubiquiti UniFi FlexHD

ramips: add support for Ubiquiti UniFi FlexHD

Hardware
--------

- SoC: MediaTek MT7621AT with 128 MiB RAM and 32 MiB Flash
- Wi-Fi: MediaTek MT7603 (b/g/n, 2x2) and MediaTek MT7615 (ac, 4x4)
- Bluetooth: CSR8811 (internal USB, install kmod-bluetooth)

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

1. Connect to the booted device at 192.168.1.20 using username/password
   "ubnt".

2. Update the bootloader environment.

   $ fw_setenv devmode TRUE
   $ fw_setenv boot_openwrt "fdt addr \$(fdtcontroladdr);
     fdt rm /signature; bootubnt"
   $ fw_setenv bootcmd "run boot_openwrt"

3. Transfer the OpenWrt sysupgrade image to the device using SCP.

4. Check the mtd partition number for bs / kernel0 / kernel1

   $ cat /proc/mtd

5. Set the bootselect flag to boot from kernel0

   $ dd if=/dev/zero bs=1 count=1 of=/dev/mtdblock4

6. Write the OpenWrt sysupgrade image to both kernel0 as well as kernel1

   $ dd if=openwrt.bin of=/dev/mtdblock6
   $ dd if=openwrt.bin of=/dev/mtdblock7

7. Reboot the device. It should boot into OpenWrt.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
(cherry picked from commit 820f0c07c544652c1c53a45262a481ac5dbe139b)