Right now we have lts_build field in our version.json. That field is not used in any way.

Now that we are introducing "semi-stable" stream builds, a boolean field is not enough. We could replace lts_build with build_type that can be development, stream, or lts.

Then we can use that field to warn people if they try to upgrade to a less stable image and ask them to confirm that it's really what they want to do.