State machine has retry logic until device is attached to network.
After this CellularContext does the context activation e.g. connect.
There was no retry logic for context activation. Added logic to
CellularContext level so it's available for at and (upcoming)ril layers.
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.
Fix syncing back to at mode after ppp disconnect.
Fix AT_CellularContext flags and states to allow new connect after disconnect.
Fix that state machine is not reseted in disconnect is it's running (might be
running because of another context or new connect already started).
It was decided to retire CellularBase class which served as a pure
virtual interface class from which Cellular network stack
implementations would get inherited. However, the current view is that
we may be the only user of it so we could retire CellularBase.