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>