Fix weston failed

Fix weston failed

 * We need to add systemd udev device unit dependency to weston,
   If the system is slow udev rules is up when weston want to start,
   if the system is fast, udev rules may be down  when weston want to start.
 * TAG+="systemd" create a systemd device unit
 * ENV{SYSTEMD_WANTS}="weston.service" add dependency (Like BindsTo).

Bug-AGL: SPEC-603

Change-Id: I45d6513ea64cd071a60b53afd98d782e9300cefa
Signed-off-by: Ronan Le Martret <ronan.lemartret@iot.bzh>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10111
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Stéphane Desneux <stephane.desneux@iot.bzh>