introduce HWTIMER_SPIN_BARRIER (API change)

Boards should define HWTIMER_SPIN_BARRIER that is used to decide
whether it makes sense to set a timer and yield or call hwtimer_spin
instead.
Used by `core/hwtimer.c` and `sys/vtimer/vtimer.c`.
A default value is provided and a warning is printed when it is used.

core/hwtimer.c

@@ -122,14 +122,15 @@ void hwtimer_wait(unsigned long ticks)
 {
     DEBUG("hwtimer_wait ticks=%lu\n", ticks);
 
-    if (ticks <= 6 || inISR()) {
+    if ((ticks <= (HWTIMER_SPIN_BARRIER)) || inISR()) {
         hwtimer_spin(ticks);
         return;
     }
 
     mutex_t mutex = MUTEX_INIT;
     mutex_lock(&mutex);
-    /* -2 is to adjust the real value */
+    /* TODO: introduce HWTIMER_INSTRUCTIONS_PER_TICK?
+     * -2 is to adjust the real value */
     int res = hwtimer_set(ticks - 2, hwtimer_releasemutex, &mutex);
     if (res == -1) {
         mutex_unlock(&mutex);

core/include/hwtimer.h

@@ -35,6 +35,7 @@
 
 #include <stdint.h>
 #include "hwtimer_cpu.h"
+#include "board.h"
 
 /**
  * @brief   Number of kernel timer ticks per second
@@ -44,6 +45,18 @@
 #warning "HWTIMER_SPEED undefined. Set HWTIMER_SPEED to number of ticks per second for the current architecture."
 #endif
 
+/**
+ * @brief       Upper bound for hwtimer_spin
+ * @verbatim    Barrier below which hwtimer_spin is called instead of
+ *              setting a timer and yielding the thread
+ * @def         HWTIMER_SPIN_BARRIER
+ */
+#ifndef HWTIMER_SPIN_BARRIER
+#warning "HWTIMER_SPIN_BARRIER undefined, using default. Set HWTIMER_SPIN_BARRIER to the \
+number of ticks below which hwtimer_spin should be used for the current board."
+#define HWTIMER_SPIN_BARRIER (6)
+#endif
+
 /**
  * @brief       Convert microseconds to kernel timer ticks
  * @param[in]   us number of microseconds

sys/vtimer/vtimer.c

@@ -347,11 +347,10 @@ int vtimer_sleep(timex_t time)
     /**
      * Use spin lock for short periods.
      * Assumes that hardware timer ticks are shorter than a second.
-     * Decision based on hwtimer_wait implementation.
      */
     if (time.seconds == 0) {
         unsigned long ticks = HWTIMER_TICKS(time.microseconds);
-        if (ticks <= 6) {
+        if (ticks <= HWTIMER_SPIN_BARRIER) {
             hwtimer_spin(ticks);
             return 0;
         }