tools: fit_image: validate existence of default configuration node

tools: fit_image: validate existence of default configuration node

When a FIT image declares a default configuration via the
'configurations/default' property, it must reference a valid subnode
under the /configurations node. If the named default does not exist,
U-Boot will fail to boot the image when no explicit configuration is
provided.

This patch adds a validation step in mkimage to check that the
referenced default configuration node is present. If not, mkimage will
print an error and abort.

This helps catch malformed or outdated ITS files early at build time
instead of deferring failure to runtime.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>