security/openvpn: re-enable mbedTLS build

...now that mbedTLS metadata was fixed to show the actual situation
for mbedTLS 2.x.y, that it's either Apache License 2.0, or
GNU General Public License 2.0 or any later version.

While here, also mark the main port with mbedTLS option enabled to
record it's going to lose the mbedTLS option end of March 2022.