In spite of there being considerable documentation sometimes in the
config file, we go with CC0 because we don't want the notion of
'derived work' of a config file.
The example `settings.conf` is also CC0. Add some docs to
it while we're at it.
- Distinguish size and atleast; in percentages they mean different
things (atleast is a bit weird as a percentage, but hey).
- Fix bug in percentage calculation.
- Avoid percentage above 100.
- Add documentation in config-file.
- Non-functional as yet, WIP
- Intended for use on SBC images and similar, which need to resize
the root partition to the size of the SD card they're on.
FIXES#921
