ewaol-distro/systemd-conf: Add custom *.link file for xenbr0 network

This patch adds '98-xenbr0-inherit-mac.link' config file used for xen bridge
interface.
This config file changes the mac address policy from 'persistent' to 'none' to
allow cloning 'xenbr0' mac address from currently connected network interface
(e.g. eth0 or enP4p4s0), instead of generating a random mac address.

Issue-Id: SCM-3910
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Change-Id: I73011213ede8d15f8036a395de16adfed4297df2