Comment 8 for bug 915215

/etc/rc.local script could simply include:

if [ -d '/etc/rc.local.d' ]; then
    for F in /etc/rc.local.d/*; do
        if [ -f "$F" ] && [ -x "$F" ]; then
            "$F"
        fi
    done
fi

But it will be hard to add this now, as user might have modified /etc/rc.local already.

However a simple systemd service could implement support for /etc/rc.local.d directory.