mount-encrypted: provide umount option for shutdown

When shutting the system down, mount-encrypted can be used to clean up
all its bind mounts and devices.

BUG=None
TEST=x86-alex build, manual testing

Change-Id: I025ce8c16c55f8556d7fff45eb6ac2b7a835101a
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/21913
Reviewed-by: Elly Jones <ellyjones@chromium.org>
4 files changed