Added FIT_LOADABLE_PROP, so the user can identify an optional entry
named "loadables" in their .its configuration. "loadables" is a comma
separated list in the .its

Documentation can be found in doc/uImage.FIT/source_file_format.txt and
                              doc/uImage.Fit/multi-with-loadables.its

Signed-off-by: Karl Apsite <Karl.Apsite@dornerworks.com>
Reviewed-by: Simon Glass <sjg@chromium.org>