x.py: allow configuring the build directory

This allows configuring the directory for build artifacts, instead of having it always be ./build. This means you can set it to a constant location, letting you reuse the same cache while working in several different directories.

The configuration lives in config.toml under build.build-dir. By default, it keeps the existing default of ./build, but it can be configured to any relative or absolute path. Additionally, it allows making outputs relative to the root of the git repository using $ROOT.

src/bootstrap/bootstrap.py

@@ -332,7 +332,7 @@ class RustBuild(object):
         self.rustc_channel = ''
         self.rustfmt_channel = ''
         self.build = ''
-        self.build_dir = os.path.join(os.getcwd(), "build")
+        self.build_dir = ''
         self.clean = False
         self.config_toml = ''
         self.rust_root = ''
@@ -891,7 +891,11 @@ def bootstrap(help_triggered):
     build.clean = args.clean
 
     try:
-        with open(args.config or 'config.toml') as config:
+        toml_path = args.config or 'config.toml'
+        if not os.path.exists(toml_path):
+            toml_path = os.path.join(build.rust_root, toml_path)
+
+        with open(toml_path) as config:
             build.config_toml = config.read()
     except (OSError, IOError):
         pass
@@ -906,6 +910,9 @@ def bootstrap(help_triggered):
 
     build.check_vendored_status()
 
+    build_dir = build.get_toml('build-dir', 'build') or 'build'
+    build.build_dir = os.path.abspath(build_dir.replace("$ROOT", build.rust_root))
+
     data = stage0_data(build.rust_root)
     build.date = data['date']
     build.rustc_channel = data['rustc']