To get Wi-Sun mesh network working, also nanostack with Wi-Sun support
is needed. ws_empty_functions.c and ws_management_api.h are temporary
included here, so that wisun_tasklet will compiled without problems.
They will replaced with the official versions with next nanostack release.
Make Nanostack an OnboardNetworkInterface, implementing
add_ethernet_interface so it can use EMAC drivers.
Can now be used via EthernetInterface, and be the system's default
network stack.
Legacy support for NanostackEthernetInterface retained. Some
restructuring of mesh interface code to fit into the
OnboardNetworkStack:::Interface system.