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/interrupt.h

@@ -0,0 +1,6 @@
+#ifndef __INTERRUPT_H__
+#define __INTERRUPT_H__
+
+#include "hw_intr.h"
+
+#endif /* __INTERRUPT_H__ */