image_to_live verifies root_dev changed.
image_to_live verifies root_dev changed.

Without this, image_to_live can sometimes think that an update from the same version to same version worked when it failed.

BUG=chromium-os:9002
TEST=By hand (hardware && success only)

Review URL: http://codereview.chromium.org/6045004

Change-Id: Ic0835372a45a45785e21ba1527d478f447bcf42a

vmware/coreos-scripts,vmware/coreos-scripts,zhang0137/scripts,marineam/coreos-scripts,sigma/coreos-scripts,nicescale/scripts,fivethreeo/scripts,glevand/coreos--scripts,BugRoger/scripts,marineam/coreos-scripts,bcwaldon/coreos-scripts,nicescale/scripts,mischief/scripts,crawford/scripts,mischief/scripts,vmware/coreos-scripts,smilart/scripts,smilart/scripts,trnubo/scripts,bcwaldon/coreos-scripts,mischief/scripts,coreos/scripts,endocode/scripts,marineam/coreos-scripts,trnubo/scripts,endocode/scripts,mjg59/scripts,crawford/scripts,crawford/scripts,mjg59/scripts,BugRoger/scripts,bcwaldon/coreos-scripts,andrejro/coreos--scripts,nicescale/scripts,smilart/scripts,zhang0137/scripts,sigma/coreos-scripts,glevand/coreos--scripts,coreos/scripts,fivethreeo/scripts,endocode/scripts,mjg59/scripts,gtank/scripts,gtank/scripts,andrejro/coreos--scripts,zhang0137/scripts,gtank/scripts,trnubo/scripts,andrejro/coreos--scripts,BugRoger/scripts,fivethreeo/scripts,sigma/coreos-scripts