[battor agent] Move code for stringifying bytes into a util file

tools/battor_agent/serial_utils.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef TOOLS_BATTOR_AGENT_SERIAL_UTILS_H_
+#define TOOLS_BATTOR_AGENT_SERIAL_UTILS_H_
+
+#include <string>
+#include <vector>
+
+namespace battor {
+
+// Prints |bytes| as a space separated list of hex numbers (e.g. {'A', 'J'}
+// would return "0x41 0x4A").
+std::string CharArrayToString(const char* bytes, size_t len);
+std::string CharVectorToString(const std::vector<char> bytes);
+
+}  // namespace battor
+
+#endif  // TOOLS_BATTOR_AGENT_SERIAL_UTILS_H_