Implement write server finish

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 7386d63..c9d7ece 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -1481,8 +1481,14 @@
  */
 int ssl_tls13_write_server_finished( mbedtls_ssl_context *ssl )
 {
-    ((void) ssl);
-    return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
+    int ret;
+
+    ret = mbedtls_ssl_tls13_write_finished_message( ssl );
+    if( ret != 0 )
+        return( ret );
+
+    mbedtls_ssl_handshake_set_state( ssl, MBEDTLS_SSL_CLIENT_FINISHED );
+    return( 0 );
 }
 
 /*