> ovm*.img files after creation.
I've implemented an option in r2594:
--remove-ovm-work-files
  If overview-levels is used, mkgmap creates one additional file
  with the prefix ovm_ for each map (*.img) file.
  These files are used to create the overview map.
  With option --remove-ovm-work-files=true the files are removed
  after the overview map was created. The default is to keep the files. 

Gerd