AP_Arming: use HAL defined compass offset max if available

libraries/AP_Arming/AP_Arming.cpp

@@ -18,7 +18,10 @@
 #include <AP_Notify/AP_Notify.h>
 #include <GCS_MAVLink/GCS.h>
 
+#ifndef AP_ARMING_COMPASS_OFFSETS_MAX
+// this can also be overridden for specific boards in the HAL
 #define AP_ARMING_COMPASS_OFFSETS_MAX   600
+#endif
 #define AP_ARMING_COMPASS_MAGFIELD_MIN  185     // 0.35 * 530 milligauss
 #define AP_ARMING_COMPASS_MAGFIELD_MAX  875     // 1.65 * 530 milligauss
 #define AP_ARMING_BOARD_VOLTAGE_MIN     4.3f