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/gdb-boot.sh b/scripts/gdb-boot.sh
index dd8a34d..60c3861 100755
--- a/scripts/gdb-boot.sh
+++ b/scripts/gdb-boot.sh
@@ -1,4 +1,6 @@
 #! /bin/bash
+#
+# SPDX-License-Identifier: Apache-2.0
 
 source $(dirname $0)/../target.sh