uncrypt: split libbootloader_message_writer for reuse.

init and vold also need to write bootloader message, so
split this function from uncrypt into a separate library.

Bug: 27176738
Change-Id: If9b0887b4f6ffab6162d9cb47a6ceb7eedd60b4d

uncrypt/include/bootloader_message_writer.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2016 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 BOOTLOADER_MESSAGE_WRITER_H
+#define BOOTLOADER_MESSAGE_WRITER_H
+
+#ifdef __cplusplus
+#include <string>
+#include <vector>
+
+bool clear_bootloader_message(std::string* err);
+
+bool write_bootloader_message(const std::vector<std::string>& options, std::string* err);
+
+#else
+#include <stdbool.h>
+
+// C Interface.
+bool write_bootloader_message(const char* options);
+#endif
+
+#endif  // BOOTLOADER_MESSAGE_WRITER_H