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.nix b/scripts/imgtool.nix
index 7d750df..9ac41bc 100644
--- a/scripts/imgtool.nix
+++ b/scripts/imgtool.nix
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 #
 # Nix environment for imgtool
 #