* extra_clean argument should be a list
  Add an assert to prevent regressions.

* properly clean package conf direcories
  They are directories now, which was causing problems.

* properly clean write_interface_* tests
  We were getting these errors:

  [Errno 21] Is a directory: './driver/write_interface_oneshot'
  [Errno 39] Directory not empty: './driver/write_interface_oneshot'
  [Errno 21] Is a directory: './driver/write_interface_make'
  [Errno 39] Directory not empty: './driver/write_interface_make'

* outputdir() is better than -outputdir, as it knows how to (pre)clean
  itself.