Make better use of modem URCs

The code is too dependent on timing at present - the config command download code was a good example of this.

However, other URCs are useful too, such as network registration and packet layer IP address assignment.

The code could be reworked to use these, such as by setting flags to say the event has been received.

The config download would be relatively simple - if the URC arrives then set a flag and download the config commands just before logging out of MQTT.

The IP address URC would be useful for setting a flag to say the packet layer is up and might be more meaningful than the current early exit flag in the modem initialisation code.