Newer nettle versions are "LGPLv3+ | GPLv2+". Add 3.1.1 but also
keep version 2.7.1 since it's LGPLv2.1+
(From OE-Core rev: 1299a222653d94f5c61f8d9ce8e450bf5684242e)
Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>