boards: iotlab-m3: add xtimer tuning values

boards/iotlab-m3/include/board.h

@@ -144,6 +144,14 @@ extern "C" {
 #define LED_ORANGE_TOGGLE   (LED_ORANGE_PORT->ODR ^= (1<<LED_ORANGE_PIN))
 /** @} */
 
+/**
+ * @name xtimer tuning values
+ * @{
+ */
+#define XTIMER_OVERHEAD     6
+#define XTIMER_SHOOT_EARLY  3
+/** @} */
+
 /**
  * @brief Initialize board specific hardware, including clock, LEDs and std-IO
  */