Update prebuilt.py to handle files not existing. Add a unittest to test the case

prebuilt.py

 #!/usr/bin/python
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import datetime
 import multiprocessing
 import optparse
 import os
 import re
@@ skipping 74 matching lines @@
   """Update the key in file with the value passed.
   File format:
     key="value"
   Note quotes are added automatically
 
   Args:
     filename: Name of file to modify.
     value: Value to write with the key.
     key: The variable key to update. (Default: PORTAGE_BINHOST)
   """
-  file_fh = open(filename)
+  if os.path.exists(filename):
+    file_fh = open(filename)
+  else:
+    file_fh = open(filename, 'w+')
   file_lines = []
   found = False
   keyval_str = '%(key)s=%(value)s'
   for line in file_fh:
     # Strip newlines from end of line. We already add newlines below.
     line = line.rstrip("\n")
 
     if len(line.split('=')) != 2:
       # Skip any line that doesn't fit key=val.
       file_lines.append(line)
@@ skipping 433 matching lines @@
   if options.board:
     UploadPrebuilt(options.build_path, options.upload, version,
                    options.binhost_base_url, board=options.board,
                    git_sync=options.git_sync, key=options.key,
                    pkg_indexes=pkg_indexes,
                    sync_binhost_conf=options.sync_binhost_conf)
 
 
 if __name__ == '__main__':
   main()