use MKSQUASHFS if set in environment
removed release specific mksquashfs binaries

- by default, the build systems /usr/bin/mksquashfs will be used
- but this can be overridden by specifying MKSQUASHFS

rational:
    - was introduced due to edge case (hardy -> lucid)
    - this is only relevant on release transitions, which can be handled
      when needed manually