Addionally to having a builtin DTB provide the possibility for
the board to provide a dtb via boarddata.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

- Add rules to generate dtb files in arch/arm/dts/
- add an initcall which unflattens and probes the internal devicetree
- Add skeleton devicetree

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>