osbuild: add --json argument

`osbuild --json [ARGS]` will suppress the normal output and print its
result as JSON. For now, it only does this when it returns 0. Otherwise,
it prints the error from the latest stage.

This is useful for other tools to call it and get machine-readable
output.

osbuild/__main__.py

@@ -21,6 +21,8 @@ def main():
                         help="the directory where intermediary os trees are stored")
     parser.add_argument("-l", "--libdir", metavar="DIRECTORY", type=os.path.abspath,
                         help="the directory containing stages, assemblers, and the osbuild library")
+    parser.add_argument("--json", action="store_true",
+                        help="output results in JSON format")
     requiredNamed = parser.add_argument_group('required named arguments')
     requiredNamed.add_argument("-o", "--output", dest="output_dir", metavar="DIRECTORY", type=os.path.abspath,
                                help="provide the empty DIRECTORY as output argument to the last stage", required=True)
@@ -35,16 +37,27 @@ def main():
         pipeline.prepend_build_pipeline(build)
 
     try:
-        pipeline.run(args.output_dir, args.store, interactive=True, libdir=args.libdir)
+        pipeline.run(args.output_dir, args.store, interactive=not args.json, libdir=args.libdir)
     except KeyboardInterrupt:
         print()
         print(f"{RESET}{BOLD}{RED}Aborted{RESET}")
-        sys.exit(130)
+        return 130
     except (osbuild.StageFailed, osbuild.AssemblerFailed) as error:
         print()
         print(f"{RESET}{BOLD}{RED}{error.name} failed with code {error.returncode}{RESET}")
-        sys.exit(1)
+        if args.json:
+            print(error.output)
+        return 1
+
+    if args.json:
+        json.dump({
+            "tree_id": pipeline.tree_id,
+            "output_id": pipeline.output_id,
+        }, sys.stdout)
+        sys.stdout.write("\n")
+
+    return 0
 
 
 if __name__ == "__main__":
-    main()
+    sys.exit(main())