ublox: Add CDC-ECM support for SARA/LISA-U2xx

The SARA-U2xx and LISA-U20x require the PDP context to be activated via +UCEDATA instead of +CGACT=1 when using the CDC-ECM USB profile. Furthermore, they expect the DHCP discover packet from the TE while the command is running, i.e. while the modem has not replied yet. In order to achieve that, the bearer is marked as connected as soon as the command is issued. This makes the NetworkManager start the DHCP process. If the modem replies with an error, the bearer will be marked disconnected later on.

I am not really sure how to test the case where the modem returns an error or the +UCEDATA command times out. Ideas are appreciated.