scripts: Add SPDX headers

Add SPDX headers indicating the Apache-2.0 license to the scripts in the
scripts directory.  This can be assumed due to the presence of the
Apache-2.0 LICENSE file at the top of the project.

Fixes #930

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/setup.py b/scripts/setup.py
index b567a9a..a228ea3 100644
--- a/scripts/setup.py
+++ b/scripts/setup.py
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 import setuptools
 from imgtool import imgtool_version