wget: use GnuTLS instead of OpenSSL

OpenSSL has license complications and GnuTLS is preferred, so although the
license complications don't impact wget use GnuTLS for consistency.

Also add a recommendation on ca-certificates so that https: URLs work.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>