uboot-mediatek: add support for Linksys E8450

Build U-Boot for the Linksys E8450 in order to have support for UBI.
The loader has a default environment with scripts handling the reset
button as well as fall-back to recovery firmware. If the loader comes
up without a valid environment found in UBI, it will automatically
make sure UBI is formatted and create a new environment and proceed
to load recovery firmware (either from UBI or via TFTP if recovery is
corrupted or unavailable).

If the button is held down during power-on, the yellow status LED
turns on and the bootloader environment is reset to factory defaults.
If the button is released at this point, the recovery firmware (if
existing) is loaded from UBI and booted.
If the button is continously held down even beyond the point that
the yellow LED turned on, the loader will try to load the recovery
firmware via TFTP from server 192.168.1.254, write it to UBI and
boot.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>