spl: fit: Allow the board to tell if more images must be loaded from FIT

spl: fit: Allow the board to tell if more images must be loaded from FIT

spl_fit_get_image_name() is used to get the names of the images that the
SPL must load from the FIT. It relies on the content of a property present
in the FIT. The list of images is thus statically defined in the FIT.
With this scheme, it quickly becomes hard to manage combinations of more
than a handful of images.
To address this problem, give the board driver code the opportunity to
add to the list of images. The images from the FIT property are loaded
first, and then the board_get_fit_loadable() is called to get more image
names.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>