homing: Ability to set retract speed between homing moves (#2487)

On the retract move between homing moves, use a different speed (typically a faster one).

Signed-off-by: Mike Stiemke <fandjango@gmx.de>

config/example.cfg

@@ -55,6 +55,10 @@ position_max: 200
 #   Distance to backoff (in mm) before homing a second time during
 #   homing. Set this to zero to disable the second home. The default
 #   is 5mm.
+#homing_retract_speed:
+#   Speed to use on the retract move after homing in case this should
+#   be different from the homing speed, which is the default for this
+#   parameter
 #second_homing_speed:
 #   Velocity (in mm/s) of the stepper when performing the second home.
 #   The default is homing_speed/2.