Adding OpenWrt support for Linksys MR6350

News flash: MR9000's firmware works to some extent!

Received my MR6350 this morning and spent all afternoon with it. The only way I can flash firmware on it is the "TFTP + serial method" as document in OpenWrt's table of hardware Linksys EA7500v1. The firmware to flash must bear the filename "fiat.img", which I found from U-Boot printenv. I first flashed EA8300 firmware on it but it didn't go anywhere - stuck in a "PHY0/1/2/3 single test" loop. I then flashed @reka's 21.02.0-rc3 MR9000 firmware (posting #47 of this thread), which allowed access with LuCI webUI and SSH but a reboot would lose the capability. The last firmware I flashed was @NoTengoBattery's Optimized build 3.0.0-rc3 for IPQ40xx. With @NoTengoBattery's build I configured MR6350 as a wireless AP and it actually worked. I had radio0 (2.4GHz) in the Client mode and radio1 (5GHz) in the AP mode which is not an ideal configuration because my main router's 5GHz is using channel 149 which is not in radio1' supported channels. So some work is needed here to move those higher-frequency channels currently allocated to radio2 to radio1 since MR6350 doesn't have radio2. One thing I noticed was the reset button didn't work - a GPIO pin thing, I believe.