Update license to Thread test file (ARMmbed#1483)

Add missing license to thread_extension_bootstrap_stub.c

Author: Arto Kinnunen

test/nanostack/unittest/stub/thread_extension_bootstrap_stub.c

@@ -1,8 +1,20 @@
 /*
- * Copyright (c) 2017 ARM Limited. All rights reserved.
+ * Copyright (c) 2017, Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
  */
 
-
 #include "nsconfig.h"
 #include <ns_types.h>
 #include <string.h>