tree-wide: add spdx header on all scripts and helpers

Even though many of those scripts are very simple, it is easier to include
the header than to try to say whether each of those files is trivial enough
not to require one.

src/basic/linux/update.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-
+# SPDX-License-Identifier: LGPL-2.1-or-later
 set -eu
 
 for i in *.h */*.h; do