Split payload application code into a subdirectory.

This patch splits from the main libupdate_engine code the part that
is strictly used to download and apply a payload into a new static
library, moving the code to subdirectories. The new library is divided
in two subdirectories: common/ and payload_consumer/, and should not
depend on other update_engine files outside those two subdirectories.
The main difference between those two is that the common/ tools are more
generic and not tied to the payload consumer process, but otherwise they
are both compiled together.

There are still dependencies from the new libpayload_consumer library
into the main directory files and DBus generated files. Those will be
addressed in follow up CLs.

Bug: 25197634
Test: FEATURES=test emerge-link update_engine; `mm` on Brillo.

Change-Id: Id8d0204ea573627e6e26ca9ea17b9592ca95bc23
diff --git a/common/hardware.h b/common/hardware.h
new file mode 100644
index 0000000..f1365e0
--- /dev/null
+++ b/common/hardware.h
@@ -0,0 +1,34 @@
+//
+// 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.
+//
+
+#ifndef UPDATE_ENGINE_COMMON_HARDWARE_H_
+#define UPDATE_ENGINE_COMMON_HARDWARE_H_
+
+#include <memory>
+
+#include "update_engine/common/hardware_interface.h"
+
+namespace chromeos_update_engine {
+namespace hardware {
+
+// The real HardwareInterface is platform-specific. This factory function
+// creates a new HardwareInterface instance for the current platform.
+std::unique_ptr<HardwareInterface> CreateHardware();
+
+}  // namespace hardware
+}  // namespace chromeos_update_engine
+
+#endif  // UPDATE_ENGINE_COMMON_HARDWARE_H_