sensors/vehicle_imu: accel clipping warning minor improvements
sensors/vehicle_imu: accel clipping warning minor improvements

 - no warning if accel is disabled
 - threshold increased 100 -> 1000 (only warn if severe ongoing clipping)
 - more generic warning message (vehicle isn't necessarily in air flying)
acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware