openvpn: enable using wolfSSL cryptographic API engine

Support for wolfSSL has been upstreamed to the master OpenVPN branch
in f6dca235ae560597a0763f0c98fcc9130b80ccf4 so we can use wolfSSL
directly in OpenVPN. So no more needed differnt SSL engine for OpenVPN
in systems based on wolfSSL library
Compiled && tested on ramips/mt7620, ramips/mt7621

Signed-off-by: Ivan Pavlov <AuthorReflex@gmail.com>