Use gcc 4.9.2 to cross-compile u-boot for Cubox and Hummingboard.

Also, fix a board-type detection problem that prevents booting on
hummingboard, by initializing fdt_board to something non-zero, so that it
ends up in the data segment instead of bss (and thus survives the memory
init process which includes zeroing bss and runs after board detection).

Approved by: rpaulo