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.

(cherry picked from commit d02b0675d0630a9ac66617becd9f9cfbbca9c524)