Updating NOTICE to give credit to the CrosVM BSD-3-Clause License.
Also updating copyright years
Firecracker
-Copyright 2017-2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+Copyright 2017-2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+SPDX-License-Identifier: Apache-2.0
+
+Portions Copyright 2017 The Chromium OS Authors. All rights reserved.
+Use of this source code is governed by a BSD-style license that can be
+found in the THIRD-PARTY file.
