different images can be grouped together to build a FIT configuration.
So far we only supported opening images as parts of configurations.
This patch adds support for opening images that are not part of a
configuration. This mode is used when the configuration parameter of
fit_open_image is NULL.

The main difference is in the way the RSA signature is checked. When
being part of a configuration all involved nodes (including the hash
nodes of the images, but not the image itself) are covered by the
signature, thus during opening an image only the validity of the image
data hash has to be checked. When not being part of a configuration,
the image data itself is signed and must be checked.

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