stepper: Adding support for a second_homing_speed parameter (#677)

Signed-off-by: Chris Whiteford <chris@chrisandtennille.com>

config/example.cfg

@@ -49,6 +49,11 @@ position_max: 200
 #homing_speed: 5.0
 #   Maximum velocity (in mm/s) of the stepper when homing. The default
 #   is 5mm/s.
+#second_homing_speed: 5.0
+#   Maximum velocity (in mm/s) of the stepper when performing the second
+#   stage of homing (the move off the endstop and then back towards for the
+#   second trigger).  This parameter is optional, and defaults to
+#   homing_speed/2.0
 #homing_retract_dist: 5.0
 #   Distance to backoff (in mm) before homing a second time during
 #   homing. The default is 5mm.