eudev: drop obsolete --with-firmware-path configure flag

eudev: drop obsolete --with-firmware-path configure flag

As recently discussed on the list:

http://lists.busybox.net/pipermail/buildroot/2017-January/182147.html

eudev no longer has legacy user space firmware loading and the
--with-firmware-path configure flag since the 3.1.3 release:

commit 3b717594600fa717cdf9bcfd0c7c1b703b245482
Author: Kay Sievers <kay@vrfy.org>
Date:   Mon Sep 1 09:07:49 2014 -0400

    udev: remove userspace firmware loading support

    Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
So drop it from EUDEV_CONF_OPTS to limit confusion.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>