mbedtls: enable DHE-RSA key exchange

Later OpenVPN 2.3-openssl versions only enable
TLS cipher suites with perfect forward secrecy, i.e. DHE and ECDHE
cipher suites. ECDHE key exchange is not supported by
OpenVPN 2.3-openssl, enable DHE key exchange to allow LEDE
OpenVPN 2.4-mbedtls clients to connect to such servers.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>
Reported-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reported-by: Lucian Cristian <luci@createc.ro>