allow rustfmt key in [build] section

Author: semarie

src/bootstrap/config.rs

@@ -201,6 +201,7 @@ struct Build {
     target: Vec<String>,
     cargo: Option<String>,
     rustc: Option<String>,
+    rustfmt: Option<String>, /* allow bootstrap.py to use rustfmt key */
     docs: Option<bool>,
     compiler_docs: Option<bool>,
     submodules: Option<bool>,