pbl: add sha256 and piggy verification to PBL ...

Extract the necessary functions from sha256 into a PBL headder and add a
verification function to the PBL. The function will be called before the
individual architectures decompress functions is run.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>