agl-image-weston: fix weston regression

agl-image-weston: fix weston regression

After 6ca247d (which changed how weston services work) services using
weston in agl-image-weston no longer start, saying "Display not found".
With this patch, they work again.

Change-Id: I7441ce2ece079db8f46a41b2ad3997951d5e18ba
Signed-off-by: Martin Kelly <mkelly@xevo.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9495
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
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>