[timer] Formalise the timer API

We now have two implementations for the timer API: one using the
time-of-day counter at 40:70 and one using RDTSC.  Both make use of
timer2_udelay().

src/config/defaults/pcbios.h

@@ -9,6 +9,7 @@
 
 #define IOAPI_X86
 #define PCIAPI_PCBIOS
+#define TIMER_PCBIOS
 #define CONSOLE_PCBIOS
 
 #endif /* CONFIG_DEFAULTS_PCBIOS_H */

src/config/timer.h

@@ -0,0 +1,15 @@
+#ifndef CONFIG_TIMER_H
+#define CONFIG_TIMER_H
+
+/** @file
+ *
+ * Timer configuration.
+ *
+ */
+
+#include <config/defaults.h>
+
+//#undef		TIMER_PCBIOS
+//#define		TIMER_RDTSC
+
+#endif /* CONFIG_TIMER_H */