package/network-manager: fix legal-info following the 1.20.0 bump

package/network-manager: fix legal-info following the 1.20.0 bump

In commit aee34c479a5f845ed84ca0fac60250843742bec6, network-manager
was bumped to 1.20.0. As part of this bump, the libnm-util/ subfolder
was removed, and therefore libnm-util/COPYING no longer exists,
breaking the legal-info of this package.

The only remaining COPYING file is at the top-level. However, it
documents only the GPL-2.0 license, while in fact many parts of
network-manager, especially libraries, are under LGPL-2.0. This is
well explained in the CONTRIBUTING file, which states:

"""
Legal
-----

NetworkManager is partly licensed under terms of GNU Lesser General Public License
version 2 or later (LGPL-2.0+). That is for example the case for libnm.
For historical reasons, the daemon itself is licensed under terms of GNU General
Public License, version 2 or later (GPL-2.0+). See the license comment in the source
files.
Note that all new contributions to NetworkManager MUST be made under terms of
LGPL-2.0+, that is also the case for parts that are currently licensed GPL-2.0+.
The reason for that is that we might eventually relicense everything as LGPL and
new contributions already must agree with that future change.
"""

So, we keep GPL-2.0 and LGPL-2.0 as license, drop libnm-util/COPYING
from the LICENSE_FILES variable, and instead add the CONTRIBUTING
file. We also add a comment above the LICENSE variable.

Fixes:

  http://autobuild.buildroot.net/results/44ce11d110a508821f76aae7ce996c1814c6a453/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>