configure: use workspace-relative path to tf_configure_bazelrc

/.bazelrc is not gitignored anymore so this should help in case the
import line is accidentally committed. Bazel 0.18.0 will support a new
'try-import' statement that should be used once 0.18.0 has been out long
enough.

Signed-off-by: Jason Zaman <jason@perfinion.com>
diff --git a/configure.py b/configure.py
index 55fce8b..129d9c5 100644
--- a/configure.py
+++ b/configure.py
@@ -257,11 +257,7 @@
       if _TF_BAZELRC_FILENAME in l:
         continue
       f.write('%s\n' % l)
-    if is_windows():
-      tf_bazelrc_path = _TF_BAZELRC.replace('\\', '/')
-    else:
-      tf_bazelrc_path = _TF_BAZELRC
-    f.write('import %s\n' % tf_bazelrc_path)
+    f.write('import %%workspace%%/%s\n' % _TF_BAZELRC_FILENAME)
 
 
 def cleanup_makefile():