Draft: ublox: Implement initial eps bearer

Implement default intial eps bearer for ublox toby-l2 and mpci-l2.

The implementation uses the +ucgdflt command to query and set the default initial eps bearer setting. The command is also used to get the initial eps bearer currently used. To use this initial bearer then, the CID (4) used by these modems to set up the initial connection, is preferred when activating a connection.