stepper: Default to a high direction pin meaning positive direction

Invert the default meaning of the stepper direction pin.  Instead of
treating a low value as position motion, treat a high value as
positive motion.  This matches what other firmwares do, and it matches
what common stepper motor drivers document.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example.cfg

@@ -19,8 +19,8 @@
 [stepper_x]
 step_pin: ar29
 #   Step GPIO pin (triggered high)
-dir_pin: ar28
-#   Direction GPIO pin (low indicates positive direction)
+dir_pin: !ar28
+#   Direction GPIO pin (high indicates positive direction)
 enable_pin: !ar25
 #   Enable pin (default is enable high; use ! to indicate enable low)
 step_distance: .0225
@@ -86,7 +86,7 @@ position_max: 200
 # stepper_x section
 [stepper_z]
 step_pin: ar23
-dir_pin: ar22
+dir_pin: !ar22
 enable_pin: !ar25
 step_distance: .005
 max_velocity: 250

config/makergear-m2-2012.cfg

@@ -5,7 +5,7 @@
 
 [stepper_x]
 step_pin: PC0
-dir_pin: PL1
+dir_pin: !PL1
 enable_pin: !PA7
 step_distance: .0225
 max_velocity: 500
@@ -20,7 +20,7 @@ position_max: 200
 
 [stepper_y]
 step_pin: PC1
-dir_pin: !PL0
+dir_pin: PL0
 enable_pin: !PA6
 step_distance: .0225
 max_velocity: 500
@@ -35,7 +35,7 @@ position_max: 250
 
 [stepper_z]
 step_pin: PC2
-dir_pin: PL2
+dir_pin: !PL2
 enable_pin: !PA5
 step_distance: .005
 max_velocity: 250
@@ -51,7 +51,7 @@ position_max: 200
 
 [extruder]
 step_pin: PC3
-dir_pin: !PL6
+dir_pin: PL6
 enable_pin: !PA4
 step_distance: .004242
 max_velocity: 200000