boards/atmega328p: Fixed xtimer frequency

The ATmega328p on a breadboard can have various frequencies depending on
the fuse settings and whether and which crystal is connected. Thus, finding a
fixed xtimer frequency that can be configured for every CPU core clock with the
available prescalers is not possible. Therefore, the fixed frequency is replaced
by a frequency depending on the CPU clock speed. For CPU clock frequency of
more than 4 MHz a prescaler of 64 will be chosen (same as on the Arduino Uno),
for 4 MHz and less a prescaler of 8 will be chosen.

boards/atmega328p/include/board.h

@@ -24,6 +24,7 @@
 #define BOARD_H
 
 #include "cpu.h"
+#include "periph_conf.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -44,7 +45,11 @@ extern "C" {
  * @{
  */
 #define XTIMER_WIDTH                (16)
-#define XTIMER_HZ                   (250000UL)
+#if CLOCK_CORECLOCK > 4000000UL
+#define XTIMER_HZ                   (CLOCK_CORECLOCK / 64)
+#else
+#define XTIMER_HZ                   (CLOCK_CORECLOCK / 8)
+#endif
 #define XTIMER_BACKOFF              (40)
 /** @} */