BootControlStub: New class used if CreateBootControl() fail.

This can happen if running update_engine on Brillo in the emulator or
other device lacking a boot_control HAL implementation. With this
change, the update_engine process continues to run but updates won't
work.

Bug: 25012817
Change-Id: I03d516c5e9a53c893c376885cd48068cd359e902
Test: update_engine no longer crashes on brilloemulator_arm.
diff --git a/boot_control_stub.cc b/boot_control_stub.cc
new file mode 100644
index 0000000..8d43530
--- /dev/null
+++ b/boot_control_stub.cc
@@ -0,0 +1,62 @@
+//
+// Copyright (C) 2015 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.
+//
+
+#include "update_engine/boot_control_stub.h"
+
+#include <base/logging.h>
+
+using std::string;
+
+namespace chromeos_update_engine {
+
+unsigned int BootControlStub::GetNumSlots() const {
+  return 0;
+}
+
+BootControlInterface::Slot BootControlStub::GetCurrentSlot() const {
+  LOG(ERROR) << __FUNCTION__ << " should never be called.";
+  return 0;
+}
+
+bool BootControlStub::GetPartitionDevice(const string& partition_name,
+                                         Slot slot,
+                                         string* device) const {
+  LOG(ERROR) << __FUNCTION__ << " should never be called.";
+  return false;
+}
+
+bool BootControlStub::IsSlotBootable(Slot slot) const {
+  LOG(ERROR) << __FUNCTION__ << " should never be called.";
+  return false;
+}
+
+bool BootControlStub::MarkSlotUnbootable(Slot slot) {
+  LOG(ERROR) << __FUNCTION__ << " should never be called.";
+  return false;
+}
+
+bool BootControlStub::SetActiveBootSlot(Slot slot) {
+  LOG(ERROR) << __FUNCTION__ << " should never be called.";
+  return false;
+}
+
+bool BootControlStub::MarkBootSuccessfulAsync(
+    base::Callback<void(bool)> callback) {
+  // This is expected to be called on update_engine startup.
+  return false;
+}
+
+}  // namespace chromeos_update_engine