Modified rootdev to handle stacked device mappers

Rootdev only went down one level when traversing the device tree.
With the addition of the bootcache device mapper, we need to
go multiple levels for the -s option for rootdev.

BUG=chromium-os:25441
TEST=used it with bootcache to find boot device.

Change-Id: Ica82dc150e403d0e49e4d8074c0b920b20e4cccc
Reviewed-on: https://gerrit.chromium.org/gerrit/31851


Commit-Ready: Paul Taysom <taysom@chromium.org>
Reviewed-by: Paul Taysom <taysom@chromium.org>
Tested-by: Paul Taysom <taysom@chromium.org>