Plane: servos move when Booting with BRD_SAFETYENABLE=0

#### Issue details
undesirable servo movements at bootup

#### Version
master

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ x ] Plane
[ ] Rover
[ ] Submarine

#### Airframe type
quadplane_tilttrivec (E-Flite Convergence)

#### Hardware type
pixracer

#### Logs

log of servo outputs from Pixracer with BRD_SAFETYENABLE=0
RCOU.Cn are scaled by .01 (scaled range [100,200])
elevons are channels 2,4, starting PWM values are near 1900
vehicle is stationary until about 38 seconds
elevons take about 35 seconds to settle down


The issue names no source file, test, or entry point. Start by reproducing Plane boot with BRD_SAFETYENABLE=0 on the Pixracer and quadplane_tilttrivec airframe, then inspect the startup servo-output behavior shown in the log; done means the elevons do not move undesirably and settle during boot.

Written by the indexing model from the issue text.