I use the hold menu key on boot (but only when the message appears). At that point the boot menu appears
EVNAME=event1 # default, n770 grep -q RX-34 /proc/component_version && EVNAME=event2 #n800 grep -q RX-44 /proc/component_version && EVNAME=event2 #n810 #wait 1 second for keyup event GETKEY="evkey -u -t 1000 /dev/input/${EVNAME}"
chroot /mnt/initfs
$GETKEY
echo $GETKEY evkey -u -t 1000 /dev/input/event2
cat /proc/component_version