arm: don't allow to select BUILTIN_DTB when building multiimage
It doesn't make sense to allow building in a single DTB into a
multiimage barebox, so we can spare the user this option.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>