This is to allow other types of PHY drivers than just RF.
Mesh-API does not actually care about driver type, it is drivers
responsibility to register right handlers with Nanostack.
* Implement a wrapper class NanostackRfPhy to ensure backward
compatibility.
* Remove mesh_connect()/disconnect() functions from MeshInterface
This job is already done in inherited classes.
* LoWPANNDInterface and ThreadInterface should only be used with
NanostackRfPhy.
Seppo Takalo