Convert license file to Markdown syntax

LICENSE.md replaces the former COPYING. It's an exact copy of the
original at https://www.gnu.org/licenses/old-licenses/gpl-2.0.md


(MD5 97bd67b5d0309e452b637f76e1c9a23c), retrieved as of Oct 22nd 2016:

HTTP/1.1 200 OK
Date: Sat, 22 Oct 2016 13:45:28 GMT
Last-Modified: Sat, 06 Aug 2016 17:46:02 GMT
ETag: "45e5-5396ac1a9ca80"
Content-Length: 17893
Content-Language: en

As github (auto)displays a license link and we also point to it
via a badge in the README.md, it's convenient to have Markdown
syntax display this file nicely formatted.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

README.md

 # sunxi-tools
-[![License](http://img.shields.io/badge/License-GPL-green.svg)][COPYING]
+[![License](http://img.shields.io/badge/License-GPL-green.svg)](LICENSE.md)
 [![Build Status](https://travis-ci.org/linux-sunxi/sunxi-tools.svg?branch=master)](https://travis-ci.org/linux-sunxi/sunxi-tools)
 
 Copyright (C) 2012  Alejandro Mery <amery@geeks.cl>
@@ -89,7 +89,7 @@ To build this, get a toolchain and run:
 
 ## License
 This software is licensed under the terms of GPLv2+ as defined by the
-Free Software Foundation, details can be read in the [COPYING][] file.
+Free Software Foundation, details can be read in the [LICENSE.md](LICENSE.md)
+file.
 
-[copying]: COPYING
 [fel mode]: http://linux-sunxi.org/FEL