add API (tos_task_create_dyn & tos_task_walkthru & tos_task_curr_task_get)

tos_task_create_dyn: create task with dynamic allocated task handler and stack
tos_task_walkthru: API to walk through the statistic list of all the existing task
tos_task_curr_task_get: get current running task handler

kernel/core/tos_sys.c

@@ -268,6 +268,10 @@ __STATIC__ void knl_idle_entry(void *arg)
     arg = arg; // make compiler happy
 
     while (K_TRUE) {
+#if TOS_CFG_TASK_DYNAMIC_CREATE_EN > 0u
+        task_free_all();
+#endif
+
 #if TOS_CFG_PWR_MGR_EN > 0u
         pm_power_manager();
 #endif