This switches the bootm code to the new uimage code. Also
bootm can now handle other types of images than uImages.
Currently the only architecture making use of this is
arm which allows to boot zImages, raw images and barebox
images.
I intended to make a more bisectable series from this but
I failed becuase there are many dependencies and no matter
how I tried the patches grew bigger and and bigger. So I
decided to put this all in a single patch.

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