Param system: Introduce global parameter version param

src/modules/systemlib/system_params.c

 /****************************************************************************
  *
- *   Copyright (c) 2013 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2013-2015 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -97,3 +97,15 @@ PARAM_DEFINE_INT32(SYS_RESTART_TYPE, 2);
 * @group System
 */
 PARAM_DEFINE_INT32(SYS_COMPANION, 0);
+
+/**
+* Parameter version
+*
+* This monotonically increasing number encodes the parameter compatibility set.
+* whenever it increases parameters might not be backwards compatible and
+* ground control stations should suggest a fresh configuration.
+*
+* @min 0
+* @group System
+*/
+PARAM_DEFINE_INT32(SYS_PARAM_VER, 1);