oeinstaller: do not use diskpart

diskpart does not work as should on usb drives
  so we just format the target drive now

  if a user, for some reason, has a 'partitioned' usb stick, it
  is not our job to clean up the mess. in this case using
  "HP USB Disk Storage Format Tool" is recommended, people should
  do it anyway when using create_installstick.bat on
  sticks with partitions