imagebuilder: unset BINARY_FOLDER and DOWNLOAD_FOLDER in final archive

imagebuilder: unset BINARY_FOLDER and DOWNLOAD_FOLDER in final archive

Using these config-options to customize the folders used at build-time
makes these folder settings appear in generated archive. This causes the
imagebuilder to be not portable, as it's going to use the build-time folders
on the new systems. Errors look like:

  mkdir: cannot create directory '/mnt/build': Permission denied
  Makefile:116: recipe for target '_call_image' failed
  make[2]: *** [_call_image] Error 1
  Makefile:241: recipe for target 'image' failed
  make[1]: *** [image] Error 2

The build-time settings of these folders are passed into the archives via
.config file.
The expected behavior is that after unpacking the imagebuilder acts like
these settings have their defaults, using intree folders. So unset the
build-time settings.

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>