Enable building cassini images for non trusted service using platforms

Only add the test user to the teeclnt group if the machine uses trusted services as if trusted services are not included the teeclnt group is not created.
It should be possible to build a cassini based test image including the trusted services test Linux user space test applications so the CI/CD flow can be enabled before the machine runs trusted services.
Override the compatible machine for the libts package so it can be built and included in images that don't have trusted service ports yet. This allows building parsec with the libts back-end.
When parsec is only built to use the trusted service back-end in becomes dependent on libts being present so we add this dependency into the parsec recipe.

Signed-off-by: Drew Reed Drew.Reed@arm.com