single shot timer interrupts fix

- accidentaly this wasn't part of the SMP merge and the implementation
  remained uncomplete with the timer keeping ticking periodically

- APIC timer is set for a signel shot and restarted everytime it
  expires. This way we can keep the AP's trully idle

- the timer is restarted a little later before leaving to userspace

- LAPIC_TIMER_ICR is written before LAPIC_LVTTR so the newest value is
  used

kernel/arch/i386/apic.c

@@ -508,14 +508,14 @@ PUBLIC void lapic_set_timer_one_shot(const u32_t usec)
 
 	ticks_per_us = (lapic_bus_freq[cpu] / 1000000) * config_apic_timer_x;
 
+	lapic_write(LAPIC_TIMER_ICR, usec * ticks_per_us);
+
 	lvtt = APIC_TDCR_1;
 	lapic_write(LAPIC_TIMER_DCR, lvtt);
 
 	/* configure timer as one-shot */
 	lvtt = APIC_TIMER_INT_VECTOR;
 	lapic_write(LAPIC_LVTTR, lvtt);
-
-	lapic_write(LAPIC_TIMER_ICR, usec * ticks_per_us);
 }
 
 PUBLIC void lapic_set_timer_periodic(const unsigned freq)

kernel/arch/i386/arch_clock.c

@@ -124,8 +124,8 @@ PUBLIC int init_local_timer(unsigned freq)
 	/* if we know the address, lapic is enabled and we should use it */
 	if (lapic_addr) {
 		unsigned cpu = cpuid;
-		lapic_set_timer_periodic(freq);
 		tsc_per_ms[cpu] = div64u(cpu_get_freq(cpu), 1000);
+		lapic_set_timer_one_shot(1000000/system_hz);
 	} else
 	{
 		BOOT_VERBOSE(printf("Initiating legacy i8253 timer\n"));