Added support for Duet 0.8.5 with auto detection between 0.6 and 0.85
board types
Added P parameter to M115 command to set board type
Changed M115 output to report the board type that was configured or
auto-detected
Improved ISR and step pulse generation efficiency to allow higher
movement speeds, especially wheh using 0.9deg/step motors
Added XYZE parameters to M569 command to allow driver remapping
Added R parameter to M569 command to allow enable signal to be reversed
when using external drivers (thanks dnewman)
Removed M558 R parameter because boare type 0.7 can now be set via M115
instead
Moved Fan0 RPM sense pin to PA14 to avoid conflict with Duet 0.8.5 FAN1
pin
M408 poll command can now be handled concurrently with other commands
