Adding support for partner-binary-overlay to cros_overlay_list

Change-Id: Iccb6f03cad814061fad18f8435e85e1bb08fc113

R=nsanders@chromium.org,scottz@chromium.org
BUG=None.
TEST=Verified overlay is listed when private-overlays/partner-binary-overlay
directory present when private overlays are requested. No errors if directory
is missing.

Review URL: http://codereview.chromium.org/6758009
1 file changed