sim: Test ram-loading with DIRECT_XIP

Combine all of the pieces to be able to test ram loading with multiple
images.  We construct a model `RamData` that indicates where each image
should be loaded into RAM.  This has to be made early enough to compute
signatures on the images, and then used later to verify that the images
were loaded properly.

Signed-off-by: David Brown <david.brown@linaro.org>