bootm: add initial FIT support ...

This implementation is inspired by U-Boot's FIT support. Instead of
using libfdt (which does not exist in barebox), configuration signatures
are verified by using a simplified DT parser based on barebox's own
code.

Currently, only signed configurations with hashed images are supported,
as the other variants are less useful for verified boot. Compatible FIT
images can be created using U-Boot's mkimage tool.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>