Add STEPPER_RESET_PIN support

Marlin/Conditionals.h

   #define HAS_MICROSTEPS_E0 (PIN_EXISTS(E0_MS1))
   #define HAS_MICROSTEPS_E1 (PIN_EXISTS(E1_MS1))
   #define HAS_MICROSTEPS_E2 (PIN_EXISTS(E2_MS1))
+  #define HAS_STEPPER_RESET (PIN_EXISTS(STEPPER_RESET))
   #define HAS_X_ENABLE (PIN_EXISTS(X_ENABLE))
   #define HAS_X2_ENABLE (PIN_EXISTS(X2_ENABLE))
   #define HAS_Y_ENABLE (PIN_EXISTS(Y_ENABLE))

Marlin/Marlin_main.cpp

 }
 
 /**
+ * Stepper Reset (RigidBoard, et.al.)
+ */
+#if HAS_STEPPER_RESET
+  void disableStepperDrivers() {
+    pinMode(STEPPER_RESET_PIN, OUTPUT);
+    digitalWrite(STEPPER_RESET_PIN, LOW);  // drive it down to hold in reset motor driver chips
+  }
+  void enableStepperDrivers() { pinMode(STEPPER_RESET_PIN, INPUT); }  // set to input, which allows it to be pulled high by pullups
+#endif
+
+/**
  * Marlin entry-point: Set up before the program loop
  *  - Set up the kill pin, filament runout, power hold
  *  - Start the serial port
   setup_killpin();
   setup_filrunoutpin();
   setup_powerhold();
+
+  #if HAS_STEPPER_RESET
+    disableStepperDrivers();
+  #endif
+
   MYSERIAL.begin(BAUDRATE);
   SERIAL_PROTOCOLLNPGM("start");
   SERIAL_ECHO_START;
     SET_OUTPUT(CONTROLLERFAN_PIN); //Set pin used for driver cooling fan
   #endif
 
+  #if HAS_STEPPER_RESET
+    enableStepperDrivers();
+  #endif
+
   #ifdef DIGIPOT_I2C
     digipot_i2c_init();
   #endif