OpenSSL: Add SHA256 support in openssl_tls_prf() for TLSv1.2

OpenSSL: Add SHA256 support in openssl_tls_prf() for TLSv1.2

This is needed when enabling TLSv1.2 support for EAP-FAST since the
SSL_export_keying_material() call does not support the needed parameters
for TLS PRF and the external-to-OpenSSL PRF needs to be used instead.

Signed-off-by: Jouni Malinen <j@w1.fi>