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/ecdsa.py b/scripts/imgtool/keys/ecdsa.py
index 81aa321..e45492b 100644
--- a/scripts/imgtool/keys/ecdsa.py
+++ b/scripts/imgtool/keys/ecdsa.py
@@ -2,6 +2,8 @@
 ECDSA key management
 """
 
+# SPDX-License-Identifier: Apache-2.0
+
 from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import ec