SMP - trully idle APs

- any cpu can use smp_schedule() to tell another cpu to reschedule

- if an AP is idle, it turns off timer as there is nothing to
  preempt, no need to wakeup just to go back to sleep again

- if a cpu makes a process runnable on an idle cpu, it must wake it up
  to reschedule

kernel/clock.h

@@ -10,7 +10,10 @@ _PROTOTYPE(int app_cpu_init_timer, (unsigned freq));
 _PROTOTYPE(int timer_int_handler, (void));
 
 _PROTOTYPE(int arch_init_local_timer, (unsigned freq));
+/* sto p the local timer ticking */
 _PROTOTYPE(void arch_stop_local_timer, (void));
+/* let the time tick again with the original settings after it was stopped */
+_PROTOTYPE(void arch_restart_local_timer, (void));
 _PROTOTYPE(int arch_register_local_timer_handler, (irq_handler_t handler));
 
 _PROTOTYPE( u64_t ms_2_cpu_time, (unsigned ms));