add rt_tick_set function

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1551 bbd45198-f89e-11dd-88c7-29a3b14d5316

include/rtthread.h

@@ -63,6 +63,7 @@ void rt_object_put_sethook(void (*hook)(struct rt_object* object));
  */
 void rt_system_tick_init(void);
 rt_tick_t rt_tick_get(void);
+void rt_tick_set(rt_tick_t tick);
 void rt_tick_increase(void);
 rt_tick_t rt_tick_from_millisecond(rt_uint32_t ms);