Add code to support sensors that allow the IR output to be modulated, to
reduce the sensitivity to ambient IR. Use M558 P2 command to enable
sensor modulation. Also changed the sensor reading averaging code to
give more consistent z-height seeking.
This change approximately doubles the speed of SD-card uploading via USB
to 2.2Mbyte/sec on the test system, provided that Pronterface has been
patched to not wait for an acknowledgement after sending each line of
g-code.