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