When the lab updated servo to R50-8041.0.0, servod for
various chromeboxes began failing.

This bug is separate from  bug 594163 , although it occurred
at the same time, and may have been caused by the same CL
As with the other bug, it will block update of servo in the
test lab.

I spot-checked for zako and panther, and saw the same
error.  Boards that may be affected include:
    zako
    panther
    guado
    tricky
    monroe
    buddy
    sumo
    mccloud
    tidus
    stumpy

Here's what I think are the relevant messages from a servo for
a zako board:
2016-03-11 12:44:11,969 - Servod - INFO - Initialized ec_ec3po_interp_connect to on
2016-03-11 12:44:11,971 - root - INFO - No device path specified for usbkm232 handler. Use the servo atmega chip to handle.
2016-03-11 12:44:13,500 - Servod - INFO - USBKM232: /dev/ttyO4
2016-03-11 12:44:14,010 - root - ERROR - Presence check response from atmega KB emu: rsp: 
Traceback (most recent call last):
  File "/usr/lib/python-exec/python2.7/servod", line 9, in <module>
    load_entry_point('servo===0.0.1-4c7d647', 'console_scripts', 'servod')()
  File "/usr/lib/python2.7/site-packages/servo/servod.py", line 512, in main
    main_function()
  File "/usr/lib/python2.7/site-packages/servo/servod.py", line 502, in main_function
    servod.hwinit(verbose=True)
  File "/usr/lib/python2.7/site-packages/servo/servo_server.py", line 772, in hwinit
    self._keyboard = self._init_keyboard_handler(self, self._board)
  File "/usr/lib/python2.7/site-packages/servo/servo_server.py", line 160, in _init_keyboard_handler
    return keyboard_handlers.USBkm232Handler(servo, self._usbkm232)
  File "/usr/lib/python2.7/site-packages/servo/keyboard_handlers.py", line 618, in __init__