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>

arch/x86/cpu/coreboot/coreboot.c

@@ -77,7 +77,8 @@ int last_stage_init(void)
 		timestamp_add_to_bootstage();
 
 	/* start usb so that usb keyboard can be used as input device */
-	usb_init();
+	if (CONFIG_IS_ENABLED(USB_KEYBOARD))
+		usb_init();
 
 	board_final_cleanup();