Create a new file named 'barebox-flash-images' in the top level output
directory that lists each image generated, one per line.  Paths will
be relative to the top level output directory.  This works if multiple
images are generated as well as for a single image.

Also update the existing barebox-flash-image symlink to point to the
image in all cases where there is a single image generated.  If
multiple images are generated, it will point to the non-existent file
'multi-image-build'.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>