- Productive embedded targets don't use the posix based variant anyway.
- For testing purposes the prefix is a hurdle as the documentation doesn't mention it ;-))
- The prefix is incompatible with posix implementations lacking directory support (like the C64).
This change means that one needs to select _ONE_ cfs implementation at Contiki library link time. But this doesn't appear to be an issue as all platforms have their "favorite" implementation anyway.