boot script is now mandatory with hwpack v2

When using hwpack v1 for VExpress, the field boot_script is None, but the same behavior with v2 results in a calltrace:

I: Saving copy of build setup to build results directory.
Traceback (most recent call last):
  File "/usr/bin/linaro-hwpack-create", line 62, in <module>
    args.CONFIG_FILE, args.VERSION, args.local_debs)
  File "/usr/lib/pymodules/python2.6/linaro_image_tools/hwpack/builder.py", line 91, in __init__
    self.config.validate()
  File "/usr/lib/pymodules/python2.6/linaro_image_tools/hwpack/config.py", line 140, in validate
    self._validate_boot_script()
  File "/usr/lib/pymodules/python2.6/linaro_image_tools/hwpack/config.py", line 607, in _validate_boot_script
    self.MAIN_SECTION)
linaro_image_tools.hwpack.config.HwpackConfigError: No boot_script in the [hwpack] section