[image] Allow for non-executable image formats

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/image.c

 	/* Check that this image can be executed */
 	if ( ( rc = image_probe ( image ) ) != 0 )
 		return rc;
+	if ( ! image->type->exec )
+		return -ENOEXEC;
 
 	/* Mark image as selected */
 	image->flags |= IMAGE_SELECTED;