images: always build sha256sum into pbl ...

Create a sha256sum of the compressed barebox image and always add it to
the PBL. We also add a custom linker section for ARM, to retrieve the
sha256sum for piggydata verification.

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