Support for device environment variables

Hey @WillFG, apologies for the delay – while it might seem that we only need to change the code to refer to the field appropriately, that would break config vars on balenaCloud (balenaCloud is now built on top of openBalena). So we’re working on refactoring a few things and moving them to open-balena which will properly fix the issue and also allow us to do other cool things such as being able to transparently introduce new API versions while maintaining compatibility with older ones. It’s a bunch of work but we’re getting there.