Add crossystem loc_idx, from ToT to R12 factory branch.

Cherry-Picked URL: http://codereview.chromium.org/6826057
                   http://codereview.chromium.org/6826014

BUG=chromium-os:14069
TEST=manual

crossystem loc_idx=3
crossystem loc_idx    # prints 3
crossystem loc_idx=0
crossystem loc_idx    # prints 0

Change-Id: I185b6d45f4c735754553b2a065ce842641741504
Reviewed-on: http://gerrit.chromium.org/gerrit/1618
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>