uboot-phytec

image: Check hash-nodes when checking configurations

It is currently possible to use a different configuration's signature and
thus bypass the configuration check. Make sure that the configuration node
that was hashed matches the one being checked, to catch this problem.

Also add a proper function comment to fit_config_check_sig() and make it
static.

Signed-off-by: Simon Glass <sjg@chromium.org>