sandbox: image: Add support for booting images in sandbox

Much of the image code uses addresses as ulongs and pointers interchangeably,
casting between the two forms as needed.

This doesn't work with sandbox, which has a U-Boot RAM buffer which is
separate from the host machine's memory.

Adjust the cost so that translating from a U-Boot address to a pointer uses
map_sysmem(). This allows bootm to work correctly on sandbox.

Note that there are no exhaustive tests for this code on sandbox, so it is
possible that some dark corners remain.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de> (v1)