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

tools/battor_agent/serial_utils.cc

+// 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.
+
+#include "tools/battor_agent/serial_utils.h"
+
+namespace battor {
+
+std::string CharVectorToString(const std::vector<char> data) {
+  std::string s;
+
+  // Reserve enough bytes for '0x', the two data characters, a space, and a null
+  // terminating byte.
+  char num_buff[6];
+  for (char d : data) {
+    // We use sprintf because stringstream's hex support wants to print our
+    // characters as signed.
+    sprintf(num_buff, "0x%02hhx ", d);
+    s += num_buff;
+  }
+
+  return s.substr(0, s.size() - 1);
+}
+
+std::string CharArrayToString(const char* bytes, size_t len) {
+  return CharVectorToString(std::vector<char>(bytes, bytes + len));
+}
+
+}  // namespace battor