mkimage: Explain the auto-fit imagefile special case

There is a special case in the code when auto-fit is used. Add a comment to
make it easier to understand why this is needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

tools/mkimage.c

@@ -273,6 +273,7 @@ static void process_args(int argc, char **argv)
 	 */
 	if (params.type == IH_TYPE_FLATDT) {
 		params.fit_image_type = type ? type : IH_TYPE_KERNEL;
+		/* For auto_its, datafile is always 'auto' */
 		if (!params.auto_its)
 			params.datafile = datafile;
 	} else if (type != IH_TYPE_INVALID) {