package/eudev: remove HOST_EUDEV_SYSCONFDIR

The HOST_EUDEV_SYSCONFDIR is a leftover from before eudev, when udev was
still built from systemd sources. Now that we have a separate file for
eudev and systemd, there's no need any more to have a condition there.
In addition, its usage in systemd.mk has been removed.

Remove the HOST_EUDEV_SYSCONFDIR variable and instead use /etc directly
in eudev.mk.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>