package/gnutls: make the OpenSSL compatibility library optional

Add a BR2_PACKAGE_GNUTLS_OPENSSL option, disabled by default since it is
not used by any package that depends on gnutls.

The library is licensed under GPLv3, which can be a problem for embedded
systems due to the so-called anti-tivoization clause.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
[Thomas: don't repeat the license details for the gnutls-openssl case,
simply append to them]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>