af-binder: Make weak ws-client by default (to be fixed)

The case of unicens service that can be here or not creates
problem to the binding 4a when the service is not here.
This commits makes the requirement of ws-client
socket optional. This unblocks the situation but need to
be fixed later, for example, by introducing an explicit
weak requirement.

Bug-AGL: SPEC-1151

Change-Id: I64864996032510f9540a6d1710cbac6e90889658
Signed-off-by: José Bollo <jose.bollo@iot.bzh>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12349
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>