mender-convert: MENDER_CONVERT_VERSION fails in docker if a git submodule

Added mender-convert as a git submodule of another project. Using 
docker-mender-convert means that the git history directory is outside of the docker bind mounted volume and hence fails when MENDER_CONVERT_VERSION assignment code is executed inside mender-convert script. Its doesn't cause the build to fail, just emits a failure message to stderr, but means  MENDER_CONVERT_VERSION variable is unavailable to configs if they should need it.
 
One simple fix would be to refactor the MENDER_CONVERT_VERSION assignment git code into a  function in a separate script that can be sourced, and then only set MENDER_CONVERT_VERSION inside mender-convert script if its currently unset/empty. Then in docker-mender-convert script source the same file containing the git function and add MENDER_CONVERT_VERSION as and additional docker run -env variable.
 
Looks straight-forward and happy to supply a pull request.