[kernel] Fix SMP not starting

The cpu.cpp/smp.cpp cleanup out of kernel_main missed an important call:
kernel_main never called smp::ready() to unblock the APs waiting for the
scheduler to be ready.

src/kernel/main.cpp

@@ -67,6 +67,7 @@ kernel_main(bootproto::args *args)
     interrupts_enable();
 
     scheduler *sched = new scheduler {g_num_cpus};
+    smp::ready();
 
     // Load the init server
     load_init_server(*args->init, args->modules);