... for easier maintenance. This commit only introduces the
framework. Follow up commits will update the license file.
Advantages:
* single source, multiple outputs (html, txt)
* conditional text, i.e. output will not include license
  terms of components that are not configured in the build

Change-Id: I0ffad41119228a0a26fd1b6ce19930ca6085adb2
Reviewed-on: https://gerrit.libreoffice.org/69396
Tested-by: Jenkins
Reviewed-by: Andras Timar <andras.timar@collabora.com>