Add SPDX-License-Identifier to source files

SPDX License List is a list of (common) open source
licenses that can be referred to by a “short identifier”.
It has several advantages compared to the common "license header texts"
usually found in source files.

Some of the advantages:
* It is precise; there is no ambiguity due to variations in license header
  text
* It is language neutral
* It is easy to machine process
* It is concise
* It is simple and can be used without much cost in interpreted
  environments like java Script, etc.
* An SPDX license identifier is immutable.
* It provides simple guidance for developers who want to make sure the
  license for their code is respected

See http://spdx.org for further reading.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>

Closes: #1439
Approved by: cgwalters

buildutil/tap-driver.sh

@@ -1,6 +1,8 @@
 #! /bin/sh
 # Copyright (C) 2011-2013 Free Software Foundation, Inc.
 #
+# SPDX-License-Identifier: LGPL-2.0+
+#
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
 # the Free Software Foundation; either version 2, or (at your option)