XDG-IVI: Load by default id-agent in weston

Bug-AGL: SPEC-1096

Change-Id: Iecfede123503410e1fdfa509a724cbcaf643d92b
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12273
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Thomas Rini <trini@konsulko.com>
Reviewed-by: Nobuhiko Tanibata <nobuhiko_tanibata@denso.co.jp>
Reviewed-by: Stéphane Desneux <stephane.desneux@iot.bzh>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>