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/imgtool/keys/general.py b/scripts/imgtool/keys/general.py
index f6b8a09..e8755f6 100644
--- a/scripts/imgtool/keys/general.py
+++ b/scripts/imgtool/keys/general.py
@@ -1,5 +1,7 @@
 """General key class."""
 
+# SPDX-License-Identifier: Apache-2.0
+
 import sys
 
 AUTOGEN_MESSAGE = "/* Autogenerated by imgtool.py, do not edit. */"