[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/main.cpp

@@ -200,7 +200,7 @@ kernel_main(args::header *header)
 	}
 
 	if (!has_video)
-		sched->create_kernel_task(logger_task, scheduler::max_priority-1, true);
+		sched->create_kernel_task(logger_task, scheduler::max_priority/2, true);
 	sched->create_kernel_task(stdout_task, scheduler::max_priority-1, true);
 
 	const char stdout_message[] = "Hello on the fake stdout channel\n";