[Timers] Initialise timers on first use

Timers are sometimes required before the call to initialise(), so we
cannot rely on initialise() to set up the timers before use.

Also fix a potential integer overflow issue in generic_currticks_udelay()

src/include/gpxe/timer.h

@@ -2,6 +2,7 @@
 #define GPXE_TIMER_H
 
 #include <stddef.h>
+#include <gpxe/tables.h>
 
 typedef unsigned long tick_t;