Make Nanostack C++ constructor initialise Nanostack

Nanostack object constructor didn't actually initialise Nanostack.
Nanostack initialisation was deferred until an interface was actually
attached to the stack, which generally happened at first interface
connect.

Not normally a problem, unless you're trying to make direct Nanostack
setup calls prior to connect - some applications do this, and were
relying on ThreadInterface::initialise to do Nanostack
initialisation.

Unfortunately in 5.9 ThreadInterface::initialise no longer does
initialise Nanostack immediately, because the mesh interfaces were
aligned and integrated with the Ethernet interfaces, which did
initialisation on connect().

Make the Nanostack object constructor initialise Nanostack (as the LWIP
constructor does for lwIP), so calling Nanostack::get_instance() is the
5.9 API for Nanostack initialisation.

For future work, APIs like ns_file_system_set_root_path should be
exposed as methods of Nanostack, so everything happens with a single
Nanostack::get_instance().file_system_set_root_path().

features/nanostack/nanostack-interface/Nanostack.cpp

@@ -449,7 +449,7 @@ void NanostackSocket::event_connection_reset(socket_callback_t *sock_cb)
 Nanostack::Nanostack()
     : call_event_tasklet(-1)
 {
-
+    mesh_system_init();
 }
 
 void Nanostack::call_event_tasklet_main(arm_event_s *event)