mkimage: Set the timestamp with -f auto

When creating a FIT, ensure that a timestamp is added, even if there is
no OS image present.

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

tools/mkimage.c

@@ -387,6 +387,8 @@ static int process_args(struct imgtool *itl, int argc, char **argv)
 			return usage(itl,
 				"Missing algorithm for auto-FIT with signed images (use -g)");
 	}
+	if (itl->auto_fit)
+		itl->reset_timestamp = 1;
 
 	/*
 	 * For auto-generated FIT images we need to know the image type to put