ramips: add support for Ubiquiti USW-Flex

ramips: add support for Ubiquiti USW-Flex

Hardware
--------
MediaTek MT7621AT
16M SPI-NOR Macronix MX25L12835FMI
Microchip PD69104B1 4-Channel PoE-PSE controller
TI TPS2373 PoE-PD controller

PoE-Controller
--------------

By default, the PoE outputs do not work with OpenWrt. To make them output
power, install the "poemgr" package from the packages feed.
This package can control the PD69104B1 PSE controller.

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

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

2. Add the uboot-envtools configuration file /etc/fw_env.config with the
   following content

   $ echo "/dev/mtd1 0x0 0x1000 0x10000 1" > /etc/fw_env.config

3. Update the bootloader environment.

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

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

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

   $ cat /proc/mtd

6. Set the bootselect flag to boot from kernel0

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

7. 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

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

Restore to UniFi
----------------

To restore the vendor firmware, follow the Ubiquiti UniFi TFTP
recovery guide for access points. The process is the same for
the Flex switch.

Signed-off-by: David Bauer <mail@david-bauer.net>