[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/syscalls/system.cpp

@@ -7,6 +7,8 @@
 #include "objects/thread.h"
 #include "syscalls/helpers.h"
 
+extern log::logger &g_logger;
+
 namespace syscalls {
 
 j6_status_t
@@ -29,9 +31,10 @@ system_noop()
 }
 
 j6_status_t
-system_get_log(j6_handle_t sys, j6_handle_t *log)
+system_get_log(j6_handle_t sys, char *buffer, size_t *size)
 {
-	return j6_err_nyi;
+	*size = g_logger.get_entry(buffer, *size);
+	return j6_status_ok;
 }
 
 j6_status_t