Config file: Split across multiple files

Suggested syntax:

include:
 - one_file.yaml
 - another_file.yaml

For relative paths, these should be interpreted as relative to the directory for the file containing the include list that's being parsed.

I'd suggest we start with some simple logic for merging files:

• Combine the dicts from parsing all of the included files with the current file
• If any of the files define the same top-level key (apart from include), throw an error (otherwise we have to consider how best to merge two board lists or similar).