Prefix management

In SmartBridge mode, the 6LBR is able to construct a DODAG without a subnet prefix available, this speed up DODAG creation after start up. Otherwise the DODAG creation is pending to the reception of a RA from the subnet router.
Also, prefix switching is supported in all mode, this is useful when the prefix of the whole subnet is modified or if a WSN node moves from one WSN to another. Without this switching capability, the DODAG must be reconstructed in the first scenario, and the node must be restarted in the second scenario.