[kernel] Add a 'log available' signal to block on

There was previously no good way to block log-display tasks, either the
fb driver or the kernel log task. Now the system object has a signal
(j6_signal_system_has_log) that gets asserted when the log is written
to.

src/kernel/objects/system.h

@@ -10,7 +10,7 @@ class system :
 public:
 	static constexpr kobject::type type = kobject::type::event;
 
-	inline static system * get() { return &s_instance; }
+	inline static system & get() { return s_instance; }
 
 private:
 	static system s_instance;