Added DueX2/DueX5 support
Report expansion board type in electronics string
Added Z probe type 6 (switch on E1 stop)
Support heater 6 on Duet 0.8.5 (untested, probably incomplete)
Changed M42 pin numbering, see duet3d.com wiki
Added M280 servo support
Allow separate firmware un-retract speed to be configured in M207
Allow negative extra un-retraction in M207
Allow dans ot be disabled yusing M106 P# I-1. They can then be used as
general-purpose pins (m42) or servos (M280).
Removed support for Duet WiFi prototype 1 board
Support expanson pin PB6 and use it for cooling fan tacho
