AP_Arming: loosen accelerometer consistency check on Z axis

Original author @jschall via 88f5d9f858

libraries/AP_Arming/AP_Arming.cpp

@@ -210,6 +210,9 @@ bool AP_Arming::ins_checks(bool report)
                      */
                     threshold *= 3;
                 }
+                // EKF is less sensitive to Z-axis error and Z-axis is more
+                // likely to be temperature-sensitive on our hardware
+                vec_diff.z *= 0.5f;
                 if (vec_diff.length() <= threshold) {
                     last_accel_pass_ms[i] = AP_HAL::millis();
                 }