Update back-end license to LGPLv2.1

Moving back the license of back-end to LGPLv2.1 since Wok and Kimchi don't depend
anymore on some imported code under LGPLv3.

COPYING.LGPL was updated according to http://www.gnu.org/licenses/lgpl-2.1.txt

Signed-off-by: Paulo Vital <[email protected]>

Author: Paulo Vital

COPYING

@@ -10,7 +10,7 @@ Kimchi makes use of different projects and they are placed as below:
 - the code under ui/fontello is imported from Fontello (http://fontello.com).
 
 The rest of this distribution is governed by the GNU Lesser General Public
-License version 3.
+License version 2.1.
 
 See COPYING.LGPL and COPYING.ASL2.