The images in FIT images can be opened in two different ways. They
can be either opened directly based on their names in the images/
node or as part of a configuration based on their names in the
corresponding /configuration/ node.

So far we only supported the latter. To prepare supporting the former
we return a cookie belonging to the configuration from
fit_open_configuration() which we use in fit_open_image() to refer
to the desired configuration.

While at it document fit_open_configuration().

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>