Ensure motors are stopped before a reboot. Ensure hard fault handler

doesn't use potentially unitialised data to update the motors.  Pause
for 50ms before rebooting after updating disabling the motors to ensure
the timer hardware and ESCs havea chance to react.

This commit might help with #123

src/main/flight/mixer.c

@@ -487,6 +487,13 @@ void writeAllMotors(int16_t mc)
     writeMotors();
 }
 
+void stopMotors(void)
+{
+    writeAllMotors(escAndServoConfig->mincommand);
+
+    delay(50); // give the timers and ESCs a chance to react.
+}
+
 #ifndef USE_QUAD_MIXER_ONLY
 static void airplaneMixer(void)
 {