ARM: rpi: switch to DT probe and multi-image build ...

This commit switches the RaspberryPi arch over to probe Barebox
from the builtin DT and enables multi-image builds.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>