Remove `kernel_type` enum in favour of `public` bool
Doesn't seem like we'll ever need more than two values for the publicity there so let's simplify things (and make them more generic so that both checkouts and builds can have this "attribute" named the same)
This should also (if done properly) fix datawarehouse#197 (I think) and should help with datawarehouse#217 (closed)
Also this is like a global change so I didn't know where else to put it other than cki-lib