Import update_payload library code

From https://android.googlesource.com/platform/system/update_engine
at ee67a8d57519112f62f439a8644ea497ab6898c2.

update_payload/__init__.py

@@ -0,0 +1,22 @@
+#
+# Copyright (C) 2013 The Android Open Source Project
+#
+# 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.
+#
+
+"""Library for processing, verifying and applying Chrome OS update payloads."""
+
+# Just raise the interface classes to the root namespace.
+from update_payload.checker import CHECKS_TO_DISABLE
+from update_payload.error import PayloadError
+from update_payload.payload import Payload