Stable firmware version assignment operates by reading content
from the build directory for the stable repair build.  If the
stable repair build is missing/non-existent, the entire run
fails, killing all firmware assignments.  The failure looks
like this:

Applying firmware updates:
Traceback (most recent call last):
  File "site_utils/stable_images/assign_stable_images.py", line 596, in <module>
    main(sys.argv)
  File "site_utils/stable_images/assign_stable_images.py", line 591, in main
    firmware_upgrades = _get_firmware_upgrades(updater, upgrade_versions)
  File "site_utils/stable_images/assign_stable_images.py", line 443, in _get_firmware_upgrades
    for board, version in cros_versions.iteritems()
  File "site_utils/stable_images/assign_stable_images.py", line 444, in <dictcomp>
    if board not in _FIRMWARE_UPGRADE_BLACKLIST
  File "site_utils/stable_images/assign_stable_images.py", line 420, in _get_firmware_version
    return _get_by_key_path(_read_gs_json_data(uri), key_path)
  File "site_utils/stable_images/assign_stable_images.py", line 308, in _read_gs_json_data
    json_object = json.load(sp.stdout)
  File "/usr/lib/python2.7/json/__init__.py", line 290, in load
    **kw)
  File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

This problem is affecting current version assignment runs, because
the veyron_rialto repair build has been deleted because it aged out.