add boot_get_loadables() to load listed images

Added a trimmed down instance of boot_get_<thing>() to satisfy the
minimum requierments of the added feature.  The function follows the
normal patterns set by other boot_get<thing>'s, which should make it a
bit easier to combine them all together into one boot_get_image()
function in a later refactor.

Documentation for the new function can be found in source:
  include/image.h

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