ramips: Add support for Xiaomi Mi Router(Black,R2100)

ramips: Add support for Xiaomi Mi Router(Black,R2100)

The Xiaomi Mi Router AC2100 is a *black* cylindrical router that shares many
characteristics (apart from its looks and the GPIO ports) with the 6-antenna
*white* "Xiaomi Redmi Router AC2100"

See the visual comparison of the two routers here:
https://github.com/emirefek/openwrt-R2100/raw/imgcdn/rm2100-r2100.jpg

Specification of R2100:
- CPU: MediaTek MT7621A
- RAM: 128 MB DDR3
- FLASH: 128 MB ESMT NAND
- WIFI: 2x2 802.11bgn (MT7603)
- WIFI: 4x4 802.11ac (MT7615)
- ETH: 3xLAN+1xWAN 1000base-T
- LED: Power, WAN in Yellow and Blue
- UART: On board (Don't know where is should be confirmed by anybody else)
- Modified u-boot

Hacking of official firmware process is same at both RM2100 and R2100.
Thanks to @namidairo

Here is the detailed guide Hack: https://github.com/impulse/ac2100-openwrt-guide
Guide is written for MacOS but it will work at linux.
needed packages: python3(with scapy), netcat, http server, telnet client

1. Run PPPoE&exploit to get nc and wget busybox, get telnet and wget firmware
2. mtd write openwrt-ramips-mt7621-xiaomi_mi-router-ac2100-kernel1.bin kernel1
3. nvram set uart_en=1
4. nvram set bootdelay=5
5. nvram set flag_try_sys1_failed=1
6. nvram commit
7. mtd -r write openwrt-ramips-mt7621-xiaomi_mi-router-ac2100-rootfs0.bin rootfs0

other than these I specified in here. Everything is same with:
https://github.com/openwrt/openwrt/commit/f3792690c4f0567a8965d82898295b9d50c3bb7e
Thanks for all community and especially for this device:
@Ilyas @scp07 @namidairo @Percy @thorsten97 @impulse (names@forum.openwrt.com)

MAC Locations:
WAN *:b5 = factory 0xe006
LAN *:b6 = factory 0xe000
WIFI 5ghz *:b8 = factory 0x8004
WIFI 2.4ghz *:b7 = factory 0x0004

Signed-off-by: Emir Efe Kucuk <emirefek@gmail.com>
[refactored common image bits into Device/xiaomi-ac2100, fixed From:]
Signed-off-by: Petr Štetiar <ynezz@true.cz>