mediatek: make GL-MT2500 image work with U-Boot recovery

mediatek: make GL-MT2500 image work with U-Boot recovery

Append metadata in the special GL.iNet format.
This also enables use of the web-based U-Boot recovery.

U-Boot-Recovery
===============

The GL-MT2500 provides web-based U-Boot recovery. For this, hold the
reset button pressed for 5 seconds when attaching power to the device.

The LED will blink 5 times. Release the reset button.

The OpenWrt sysupgrade image can be installed by navigating to
http://192.168.1.1 in a web-browser.

The device does not work as a DHCP server, so manual IP configuration is
required.

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