Use tls_connection_prf() for all EAP TLS-based key derivation

Use tls_connection_prf() for all EAP TLS-based key derivation

tls_openssl.c is the only remaining TLS/crypto wrapper that needs the
internal PRF implementation for EAP-FAST (since
SSL_export_keying_material() is not available in older versions and does
not support server-random-before-client case). As such, it is cleaner to
assume that TLS libraries support tls_connection_prf() and move the
additional support code for the otherwise unsupported cases into
tls_openssl.c.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>