Copter: use circular limit on tilt angle for arm check

ArduCopter/motors.pde

@@ -625,7 +625,7 @@ static bool arm_checks(bool display_failure, bool arming_from_gcs)
 
     // check lean angle
     if ((g.arming_check == ARMING_CHECK_ALL) || (g.arming_check & ARMING_CHECK_INS)) {
-        if (labs(ahrs.roll_sensor) > aparm.angle_max || labs(ahrs.pitch_sensor) > aparm.angle_max) {
+        if (degrees(acosf(ahrs.cos_roll()*ahrs.cos_pitch()))*100.0f > aparm.angle_max) {
             if (display_failure) {
                 gcs_send_text_P(SEVERITY_HIGH,PSTR("Arm: Leaning"));
             }