[Timers] Miscellaneous timer system fixes

Add missing comments to timer code.

Lock system if no suitable timer source is found.

Fix initialisation order so that timers are initialised before code that
needs to use them.

src/core/serial.c

@@ -263,6 +263,6 @@ struct startup_fn serial_startup_fn __startup_fn ( STARTUP_NORMAL ) = {
  * early debug messages.  It is safe to call serial_init() multiple
  * times.
  */
-struct init_fn serial_init_fn __init_fn ( INIT_EARLY ) = {
+struct init_fn serial_init_fn __init_fn ( INIT_CONSOLE ) = {
 	.initialise = serial_init,
 };