Add GPL license text file

Add missing license text file which is needed to package mesaflash
for Fedora linux distribution.

The following command was used to obtain the license text file:
 $ wget -O COPYING https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt



Signed-off-by: Damian Wrobel <dwrobel@ertelnet.rybnik.pl>