This disables build-args support, which is nice when building things
on e.g. a shared build-machine, where we don't want the build to
be able to break out of the sandbox (by specifying e.g. --share=network).
Stop building at a specified module from the json file.
This is useful during development. For instance, you can do --stop-at=main-app
to build all the dependencies into the appdir, then you can do a build of an
already checked out git repo (possibly with changes).
This includes a few different changes:
* Add build-runtime boolean property
* Rename "app-id" property to "id"
* Add metadata property to use a custom base metadata file
* Default to writable-sdk to TRUE for runtimes
* Default prefix to /usr for runtimes
* Put manifest in usr for runtimes
* Pick up debuginfo from usr for runtimes
* Make build-finish work on runtimes, but only export appdata
