sandbox: image: Create a test for loading FIT images

The image code is fairly complex with various different options. It would
be useful to have comprehensive tests for this.

As a start, create a script which tries out loading a kernel/ramdisk/fdt
from a FIT and checks that the images appear in the right place in memory.

This uses sandbox which now supports bootm and related features.

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