Tidying up parking override control implementation

[new] Added a default configuration for the parking override control
upon a reset or power-up. By default, parking is enabled, but this may
be disabled via a config.h option.

[fix] Parking override control should be checking if the command word
is passed, rather than the value.

grbl/report.c

@@ -328,7 +328,7 @@ void report_gcode_modes()
   #endif
 
   #ifdef ENABLE_PARKING_OVERRIDE_CONTROL
-    if (sys.override_ctrl) { 
+    if (sys.override_ctrl == OVERRIDE_PARKING_MOTION) { 
       report_util_gcode_modes_M();
       print_uint8_base10(56);
     }