bootstd: Move common zimage functions to bootm.h

bootstd: Move common zimage functions to bootm.h

We want to avoid using #ifdefs around header files and in the code. It
makes sense to collect the various functions used for loading images into
a single header which can be included by all architectures. The best place
for this is the arch-neutral bootm.h header, so use that.

Move some zimage functions into this bootm.h header.

Signed-off-by: Simon Glass <sjg@chromium.org>