openvpn: remove polarssl crypto backend option

Now that we need to bump openvpn to version 2.3.11 for security fixes
the time has come to remove the polarssl option.
Add legacy handling explaining the situation:
PolarSSL 1.2.x can coexist with mbedTLS 2.x+, but OpenVPN requires
PolarSSL/mbedTLS 1.3.x (the transition branch) >= 1.3.8 but doesn't
build/work with the 2.x series. And PolarSSL/mbedTLS 1.3.x can't coexist
with mbedTLS 2.x on the same target.
So, unfortunately, openssl is now the only option (until libressl
arrives) which means no more backend options in general.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>