Refactor a bit.

  # assert p.returncode == 0, "build system.img image failed"

  # TODO: Why the above common.Run() generate different system.img for crespo?

  str_command = "mkyaffs2image -f ";

  if OPTIONS.mkyaffs2_extra_flags is not None:

    str_command = str_command + OPTIONS.mkyaffs2_extra_flags

  str_command = str_command + " " + os.path.join(OPTIONS.input_tmp, "system") + " " + img.name

  str_command = " ".join(build_command)

  print "running " + str_command

  exit_code = os.system(str_command)

  assert exit_code == 0, "build system.img image failed"