tls13:server:add server certificate writing

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 41e7729..c9de3b4 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -1449,7 +1449,12 @@
 int ssl_tls13_write_server_certificate( mbedtls_ssl_context *ssl )
 {
     ((void) ssl);
-    return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
+    int ret = mbedtls_ssl_tls13_write_certificate( ssl );
+    if(ret != 0)
+        return( ret );
+
+    mbedtls_ssl_handshake_set_state( ssl, MBEDTLS_SSL_CERTIFICATE_VERIFY );
+    return( 0 );
 }
 
 /*