keyboard: For chromeboxes, return a None keyboard_handler if no atmega chip

When using servo-micro, which doesn't have an atmega chip inside,
the keyboard init code raises an error about atmega. We should skip
initializing it, in order not to block the development use.

BUG=b:68166165
TEST=Started 'servod -b guado' using servo-micro. Saw a warning but
other functions work fine:
  INFO - No device path specified for usbkm232 handler. Use the servo atmega chip to handle.
  WARNING - No atmega in servo board. So no keyboard support.

Change-Id: I9715fb88c46c83e1a07fc0c210efd7695a8a205b
Reviewed-on: https://chromium-review.googlesource.com/736786
Commit-Ready: Wai-Hong Tam <waihong@google.com>
Tested-by: Wai-Hong Tam <waihong@google.com>
Reviewed-by: Nick Sanders <nsanders@chromium.org>
1 file changed