coreboot: only init usb if necessary

coreboot: only init usb if necessary

Up until now the call to initialize the USB subsystem was hardcoded
for U-Boot running as a coreboot payload. This was used to enable
the use of a USB keyboard in the U-Boot shell. However not all boards
might need this functionality. As initializing the USB subsystem can
take a considerable amount of time (several seconds on some boards),
we now initialize the USB subsystem only if U-Boot is configured to
use USB keyboards.

Signed-off-by: Thomas RIENOESSL <thomas.rienoessl@bachmann.info>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>