[config] Dist supports custom super image script

Some boards need to supply a custom super image script, and this
is supported via the BOARD_CUSTOM_BUILD_SUPER_IMAGE variable.

However, this variable is not used by img_from_target_files.py.

Change img_from_target_files.py to call the supplied script.

Bug: None
Change-Id: I6bca937534f58459d3419139ee7a37a10138ab8f