- restructure the use of prebuilt boot images
  * adapt to new data structure thus we get rid of
    checkAndSetupPrebuiltBootImage, getDefaultPrebuiltDir_legacy
  * moved the logic for using a prebuilt version of a boot image
    into the image creator code at the level of prepareBootImage
    and createBootImage. This saves us one KIWIXML instance
  * write one function createImageBootImage which does either
    the prepare/create of the boot image or the use of an existing
    prebuilt version. reduces some code duplication