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)