Make bundle overlaying work for files as well as directories.
Make bundle overlaying work for files as well as directories.

BUG=None
TEST=buildbot

Change-Id: I0c356f397742a6e50fed1cc2ad98d0e2087af3b0
(cherry picked from commit 229c5373a4446de140fd52c57d6441cd58f6ce19)
Reviewed-on: https://gerrit.chromium.org/gerrit/38628
Tested-by: Jon Salz <02f606d7fe027b93db0ccb8283f0d43a8932fe7a@chromium.org>
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Commit-Ready: Jon Salz <02f606d7fe027b93db0ccb8283f0d43a8932fe7a@chromium.org>

bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,zhang0137/chromite