In some multithread cases there is possibility that process_oob function
was called after ATHandler was deleted. Fix is to wait if oob processing
is ongoing.
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.
Moved methods to classes CellularDevice and CellularInformation.
SIM interface was removed to simplify cellular usage and
methods better suite new classes.
Updated greentea and unit tests.