image_signing: fix signing of zip/crx files

Restore the search logic for manifests in subdirs.

BRANCH=None
BUG=chromium:697645
TEST=signed adb/fastboot zip archives

Change-Id: I07a417216ea463cb00d6ead7cd3b61d6e6fa507d
Reviewed-on: https://chromium-review.googlesource.com/494207
Commit-Ready: Hsinyu Chao <hychao@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Eric Caruso <ejcaruso@chromium.org>
1 file changed