arm: fill BUILTIN_DTB_NAME for boards that still use this option
There are only a few users of BUILTIN_DTB left, but those should
alsways fill thr name properly. Avoids lots of failures in the
randconfig builds.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>