tools: mkimage: validate image references in FIT configurations

tools: mkimage: validate image references in FIT configurations

When parsing a FIT image source (ITS), mkimage does not currently check
whether the image names referenced in the /configurations section (e.g.
"kernel", "fdt", "ramdisk", "loadables") actually exist in the /images
node.

This patch introduces a validation step during FIT import that iterates
over each configuration and verifies that all referenced image names are
defined under /images. If a missing image is detected, an appropriate
error is reported and mkimage exits with FDT_ERR_NOTFOUND.

This ensures that configuration integrity is validated at build time.

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