build_image: correctly label var for rootfs.

${rootfs}/var is a bind-mount from stateful/var_overlay. setfiles
will skip directories on a different device.

This change umounts ${rootfs}/var so setfiles can correctly label
var in rootfs, instead of skipping var.
During boot-time, /var is mounted as encrypted stateful partition,
and labelled as cros_var. But besides the var in encrypted stateful
partition, we also need origional var in rootfs to be correctly
labelled.

BUG=b:116072767
TEST=boot, umount /var; ls -Zd /var

Change-Id: If54e441ac786e5c3c648833ea76bf20211151f8f
Reviewed-on: https://chromium-review.googlesource.com/1270324
Commit-Ready: Qijiang Fan <fqj@google.com>
Tested-by: Qijiang Fan <fqj@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed