Add support for an optional context parameter to TLS exporter

Allow an additional context value to be passed to TLS exporter as
specified in RFC 5705 section 4.

This does not yet implement it for the internal TLS implementation.
However, as currently nothing uses context yet, this will not break
anything right now. WolfSSL maintainers also stated that they are not
going to add context support yet, but would look into it if/when this is
required by a published draft or a standard.

Signed-off-by: Ervin Oro <ervin.oro@aalto.fi>

src/crypto/tls_openssl.c

@@ -3908,11 +3908,13 @@ static int openssl_get_keyblock_size(SSL *ssl)
 
 
 int tls_connection_export_key(void *tls_ctx, struct tls_connection *conn,
-			      const char *label, u8 *out, size_t out_len)
+			      const char *label, const u8 *context,
+			      size_t context_len, u8 *out, size_t out_len)
 {
 	if (!conn ||
 	    SSL_export_keying_material(conn->ssl, out, out_len, label,
-				       os_strlen(label), NULL, 0, 0) != 1)
+				       os_strlen(label), context, context_len,
+				       context != NULL) != 1)
 		return -1;
 	return 0;
 }