Telemetry logs fill up disk

[felix-walter]

We have a setup where a MAVProxy instance is running continuously on an onboard computer attached to a Pixhawk 4. The .tlog and .tlog.raw files grow until the disk usage is at 100%. It would be great if one could either a) prevent these log files from being created completely or b) set a size limit via the command line parameters.

[stephendade]

See #1112