Provide information how ISO was generated in file conf/buildinfo.json

Provide information how ISO was generated in file conf/buildinfo.json

Relevant changes:

* Depend on "jo" tool, used for generating the json output in
  conf/buildinfo.json

* Do not rewrite $SQUASHFS_NAME content, instead handle its fixed output
  format via $fixed_squashfs_name

* Same for $RELEASE_INFO content, handled via $fixed_squashfs_name now

* Always set $SQUASHFS_BINARY if unset, to be able to log
  a proper command line, even if mksquashfs stage gets skipped

FTR: the generated file is accessible on the live system under
/run/live/medium/conf/buildinfo.json

Closes: https://github.com/grml/grml-live/issues/44