weston.service not starting on m3ulcb, rpi3

weston service check is failing on m3ulcb and rpi3.
See email report with failures: https://lists.linuxfoundation.org/pipermail/automotive-testreports/2019-August/000135.html

For m3ulcb, this is likely due to the nogfx build, so we need to use build-info to decide whether to run the service check.

For rpi3, it's probably the case that the weston service will fail to start if there is no touchscreen attached, so we will need to check device tags to decide whether to run weston.