Add the following to /etc/rc.local
. This has been verified as working on a Raspberry Pi 3 with the caveat that on power up and reboot, the LEDs are on briefly before being disabled.
for led in /sys/class/leds/*
do
printf "Disabling LED: $led\n"
sudo sh -c "echo 0 > $led/brightness"
sudo sh -c "echo none > $led/trigger"
done