image_signing: fix rootfs mount when checking kernel params

Use the existing mount helper so we clean up automatically when exiting.

BUG=None
TEST=CQ passes
BRANCH=None

Change-Id: I882c7f5ea3b54e08745c48378cc50702550cdc71
Signed-off-by: Mike Frysinger <vapier@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2923828
Reviewed-by: George Engelbrecht <engeg@google.com>
1 file changed