[ssu] Warn if registration fails due to non-writable configuration

/usr/share/ssu/kickstart/pack/<model>, with default fallback

Recursive search used to return the first match found, now return the last match
found. Additionally, fall back to an autodetected default section if required.

- variable section resolving now tries to automatically identify default
  sections when looking up a complete section
- local variables may be defined by prefixing with _ or using the local
  keyword
- multiple variable sections are now supported, as well as adding both
  the variables keyword and variable values in a section

- 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.

make sure no spaces from model are not forwarded to other places where those are not wanted.

Signed-off-by: Marko Saukko <marko.saukko@jollamobile.com>

Somehow I forgot to refactorize this in past. Available since 8b124190.