When using NetworkInterface::get_default_instance() application gets handle
to CellularInterface which is actually CellularContext derived from CellularInterface.
Application needs also handle to CellularDevice to open other interfaces.
Simplified state machine by removing manual registering state.
This was done as some modems did not have all the needed at commands
for checking the registered network. Some modem run out of memory as when
checking correct network there might be so many networks available.
Manual registration still works but it does not do any checks to which network
it's registered. Moved manual registering at command earlier in state machine so it forces
registering to a correct network. Internal refactor/fix, does not affect applications.
Antti Kauppila
Teppo Järvelin
Ari Parkkila
Mirela Chirica