LICENSES: move the GPL-2.0 license as required by checkpatch

LICENSES: move the GPL-2.0 license as required by checkpatch

The script spdxcheck.py (used by checkpatch.pl) searches for the
license files in a dedicated folder, split in the subfolders:
	"preferred", "dual", "deprecated", "exceptions".

Move the GPL-2.0 license from COPYING to the subfolder preferred,
adding the header required by spdxcheck.py.
The header is copied from equivalent file in the Linux kernel,
made by Thomas Gleixner <tglx@linutronix.de>.
Note: the license in COPYING matches exactly
	https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt

Add a minimalist COPYING in the top directory, as required by
automake, pointing to the files in LICENSES.

Change-Id: I1fd0abc57b554d74f0b00fabd6c8c822b2c4acb5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5967
Tested-by: jenkins