busybx umount does not support -O flag, but it's used in netmount script

the /etc/init.d/netmount script contains this code:

if [ “$RC_UNAME” = Linux ] && [ $retval = 0 ]; then
umount -a -O _netdev
retval=$?
fi

it shows an error when shutting down/rebooting system with attached usb disk, because -O is not supported by busybox
it is a cosmetic issue, hence low priority

(from redmine: issue id 9923, created on 2019-01-26)