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>