Provide a new API for accessing uImages which makes it easy
for commands to open images, verify them, load to (free) sdram
regions and show information about uImages.

- We now do not load the image to malloced space anymore.
- The data in the header is now stored in cpu native endianess
  after uimage_open which makes it easy to access the header data.
- uImage can be loaded to dynamically allocated sdram regions.

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