previously the start log button was always disabled, even for PX4.

Fixes regression from 9d413108

It's not desired to have automatic log streaming for several reasons:
- There's already SD logging (which is the case for the majority of the
  users)
- The link does not provide the necessary bandwidth (eg. wireless telemetry)

The return lead to data being dropped. Some of this data could have been
part of the ulog header, which lead to corrupt files.

Moving source files into Vehicles as it is a more coherent place.

Forcing QGC to use Mavlink V2 if vehicle supports it.