uboot-mediatek: add support for GL.iNet GL-MT2500

uboot-mediatek: add support for GL.iNet GL-MT2500

Add an OpenWrt-based U-Boot build for this device, allowing for more
flexibility and customization.

Expected behaviour
------------------
When plugging the device, keeping the reset button pressed will enter
TFTP recovery mode: the board will send requests for the initramfs file
(openwrt-mediatek-filogic-glinet_gl-mt2500-initramfs-kernel.bin) from
IP 192.168.1.1 to 192.168.1.10 TFTP server.
The bootloader will populate the environment with the unit serial number
as reported by the "sn" value in eMMC, the "sn_bak" value, the country
code and ddns. WAN and LAN MAC addresses are reported in the environment
as well.

Limitations
-----------
No web interface is available, and only the LAN port can be used for
system recovery.

Notes
-----
This port has been tested with the Maxlinear version only, and with a
board that does not exhibit eMMC communication problems. Even though eMMC
frequency has been lowered, some testing is probably needed and always
very welcome.
A -factory image has been introduced, and is only needed when using the
"Load Firmware via TFTP then write to eMMC." boot menu function.
The device has not been converted to use uImage.FIT including the rootfs
to keep compatibility with stock bootloader and firmware.

Installation
------------
From a Linux root shell:
0. Transfer the needed files to the board, placing them in /tmp.
1. Make sure your U-Boot environment is erased: all of my units came with
unpopulated environment, but I am not sure this is always the case.
Issue the command
cat /dev/zero >/dev/mmcblk0p2
(you will get a "no space left" error, which is reasonable and expected).
2. Unlock the eMMC boot area where BL2 is located:
echo 0 >/sys/block/mmcblk0boot0/force_ro
3. Write the new BL2 code:
cat openwrt-mediatek-filogic-glinet_gl-mt2500-emmc-preloader.bin >/dev/mmcblk0boot0
4. Write new BL31+U-Boot image:
cat openwrt-mediatek-filogic-glinet_gl-mt2500-emmc-bl31-uboot.fip >/dev/mmcblk0p4
5. Reboot.

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>