gnutls: update legal info

gnutls: update legal info

GnuTLS core library is licensed under LGPLv2.1+ while gnutls-openssl
library is licensed under GPLv3+. Annotate the license with components.

Reviewed-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
[Thomas:
 - add comment about the license of the "core library" since some files
   seems to be mistakenly under LGPLv3+ even though the library is
   licensed under LGPLv2.1+.
 - add the README file in the license information, since it contains a
   lot of useful details.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>