Add exit_thread prototype. Make sure exit_thread doesn't return by accident

diff -r 7f206dc573bc -r a7922b798098 extras/mini-os/include/sched.h
@@ -39,6 +39,7 @@ void init_sched(void);
 void init_sched(void);
 void run_idle_thread(void);
 struct thread* create_thread(char *name, void (*function)(void *), void *data);
+void exit_thread(void) __attribute__((noreturn));
 void schedule(void);
 
 #define current get_current()

diff -r 7f206dc573bc -r a7922b798098 extras/mini-os/sched.c
@@ -198,7 +198,11 @@ void exit_thread(void)
     list_add(&thread->thread_list, &exited_threads);
     local_irq_restore(flags);
     /* Schedule will free the resources */
-    schedule();
+    while(1)
+    {
+        schedule();
+        printk("schedule() returned!  Trying again\n");
+    }
 }
 
 void block(struct thread *thread)