Add SPDX-License-Identifier to source files

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