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.
ns_event_loop_thread_start() is incorrectly used at connect() phase, the initial
setup is already done at init() phase and the eventloop thread is correctly initialized.
Also, the usage of ns_event_loop_thread_start() API should be behind MBED_CONF_NANOSTACK_HAL_EVENT_LOOP_DISPATCH_FROM_APPLICATION
flag as application can decide to use main thread for event loop, which will result in linker error for this API call in
case of ARMCC compiler.