Reduce Autotalks link layer dependency

[riebl]

The current Autotalk link layer of socktap depends a lot on the Autotalks SDK which also changes its API from time to time. Due do license restrictions we cannot even compile the Autotalks link layer as part of our CI. Thus, I would like to reduce the dependency on proprietary APIs in Vanetza. For example, we could move the Autotalks code to a separate binary which exposes the V2X radio interface via UNIX domain sockets. Only the helper binary would depend on the Autotalks API then.