Refactor MkdirP out into a utility.

We want to use it again!

Change-Id: Iec72840e8fa1a8915e8176842f865a2f8acbfd93
diff --git a/aos/util/file.h b/aos/util/file.h
index 3aebd87..d6724af 100644
--- a/aos/util/file.h
+++ b/aos/util/file.h
@@ -15,6 +15,8 @@
 void WriteStringToFileOrDie(const std::string_view filename,
                             const std::string_view contents);
 
+void MkdirP(std::string_view path, mode_t mode);
+
 }  // namespace util
 }  // namespace aos