start working on config.json

#254

this is by no means complete, things that are still missing:
- config.json should rebuild when banners/flags are updated
- reverse the key/value order of the flags? (right now it's 'filename': 'description')
- add more options

any suggestions for more options to add? I don't know if adding stuff like max. file size, etc. would be beneficial if it's only set differently manually for one or two boards, so I've left that out.