[fb] Output klog to fb if video exists

If there's no video, do as we did before, otherwise route logs to the fb
driver instead. (Need to clean this up to just have a log consumer
general interface?) Also added a "scrollback" class to fb driver and
updated the system_get_log syscall.

src/kernel/log.h

@@ -6,4 +6,5 @@ namespace log = kutil::log;
 namespace logs = kutil::logs;
 
 void logger_init();
+void logger_clear_immediate();
 void logger_task();