quectel: implement DFOTA update method

Adds Quectel's DFOTA as an update method. Delta Firmware Over The Air upgrades only contain the diff between the current and target firmware version, which is required for some small storage devices.

I chose not to probe with AT+QFOTADL=? since it would either override or be overridden by the firehose and fastboot methods. As most Quectel devices support at least one of firehose and fastboot already, I decided not to include Udev rules for DFOTA devices here, so it can be selected manually when required.