[ssuks] Arbitrary command sections, support for spaces in Model, Model validation

- key 'commandSections' in 'var-kickstart-defaults' section in board-mappings
  now specifies a list of directories to search for additional command sections
  to include. Search algorithm is like for partitions. If the key is unset only
  the partition directory will get searched.
- spaces in the model name will be converted to _ (underscore) whenever spaces
  don't make sense.
- Existence of the model is now checked (and writing kickstarts for non-existant
  models needs to be forced). Generating kickstarts for model names with several
  consecutive spaces is refused.