test: spl: Add functions to create images

This add some basic functions to create images, and a test for said
functions. This is not intended to be a test of the image parsing
functions, but rather a framework for creating minimal images for testing
load methods. That said, it does do an OK job at finding bugs in the image
parsing directly.

Since we have two methods for loading/parsing FIT images, add LOAD_FIT_FULL
as a separate CI run.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>