weston: allow weston to be started from a systemd user session

weston: allow weston to be started from a systemd user session

As part of the App FW rework, we want weston to be started from a
systemd user session, which is allowed by this patch (backported from
upstream).

Bug-AGL: SPEC-4161
Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Change-Id: I23961eaebb65bdbdcfd8c3d01ff1941f978385ec
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26968
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>