ramips: add support for Linksys E5600

ramips: add support for Linksys E5600

This submission relied heavily on the work of Linksys EA7300 v1/ v2.

Specifications:

* SoC: MediaTek MT7621A (880 MHz 2c/4t)
* RAM: 128M DDR3-1600
* Flash: 128M NAND
* Eth: MediaTek MT7621A (10/100/1000 Mbps x5)
* Radio: MT7603E/MT7613BE (2.4 GHz & 5 GHz)
* Antennae: 2 internal fixed in the casing and 2 on the PCB
* LEDs: Blue (x4 Ethernet)
  Blue+Orange (x2 Power + WPS and Internet)
* Buttons: Reset (x1)
  WPS (x1)

Installation:

Flash factory image through GUI.

This device has 2 partitions for the firmware called firmware and
alt_firmware. To successfully flash and boot the device, the device
should have been running from alt_firmware partition. To get the device
booted through alt_firmware partition, download the OEM firmware from
Linksys website and upgrade the firmware from web GUI. Once this is done,
flash the OpenWrt Factory firmware from web GUI.

Reverting to factory firmware:

1. Boot to 'alt_firmware'(where stock firmware resides) by doing one of
   the following:
   Press the "wps" button as soon as power LED turns on when booting.
   (OR) Hard-reset the router consecutively three times to force it to
   boot from 'alt_firmware'.
2. To remove any traces of OpenWRT from your router simply flash the OEM
   image at this point.

Signed-off-by: Aashish Kulkarni <aashishkul@gmail.com>
[fix hanging indents and wrap to 74 characters per line,
 add kmod-mt7663-firmware-sta package for 5GHz STA mode to work,
 remove sysupgrade.bin and concatenate IMAGES instead in mt7621.mk,
 set default-state "on" for power LED]
Signed-off-by: Sannihith Kinnera <digislayer@protonmail.com>
[move check-size before append-metadata, remove trailing whitespace]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 251c995cbb1ea5ad1de14775312c2bd19ed10439)