Fix compilation error if CONFIG_USB is disabled

This patch is to fix the following compilation error when
disabling CONFIG_USB for Rpi3:

include/config_distro_bootcmd.h:242:2: error: expected ‘}’
before ‘BOOT_TARGET_DEVICES_references_USB_without_CONFIG_CMD_USB’
     BOOT_TARGET_DEVICES_references_USB_without_CONFIG_CMD_USB

Signed-off-by: Ajay Kaher <akaher@vmware.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>