Define default CONFIG_PREBOOT with right config option

The 44758771 commit removes CONFIG_PREBOOT but actually sets the USE_PREBOOT
Kconfig option which isn't CONFIG_PREBOOT and is also a bool option which means
we regress because 'usb start' isn't run when expected, it should also be run
for devices that have USB storage because keyboards aren't the only thing we
might need the USB bus for.

Fixes: 44758771 ("arm: move CONFIG_PREBOOT="usb start" to KConfig")
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Cc: Jonas Smedegaard <dr@jones.dk>
Cc: Neil Armstrong <narmstrong@baylibre.com>