image: Add FIT image loadable section custom processing

To help automate the loading of custom image types we add the ability
to define custom handlers for the loadable section types. When we find
a compatible type while loading a "loadable" image from a FIT image we
run its associated handlers to perform any additional steps needed for
loading this image.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>