Check if AUTOTRIM is enabled before arming and avoid arming.

[giacomo892]

Avoiding to ARM with AUTOTRIM enabled will prevent the plane to trim on the ground while not leveled in pilot's hands with mayor effects on the flight performance.

Also while checking this I've noted that SERVO_AUTOTRIM_TIMER_MS is set to 1000 while on the documentation it says to fly level for 5 seconds.

Should we increase it at least to 2s to allow for better trimming if there is some turbulence?

[shellixyz]

I think it is a good idea to increase the autotrim timer. 5 seconds doesn't seem too much. 1 second definitely seems too little. At least the documentation should be updated. I was surprised when I used it and it changed the controls in less than 5 seconds.

[giacomo892]

I'm doing a PR updating it to 2 seconds. With 1 second it is doing a rather good job. 5 perhaps is too long (#2769)