update_kernel: fix to be not rely on kernel command-line
update_kernel: fix to be not rely on kernel command-line

This script recently stopped working when we changed the kernel cmdline.

Fix to be more robust by using rootdev instead.

BUG=12780
TEST=Updated my kernel.

Change-Id: I033f670c5db821863441ea1546c382fce3436c01

Review URL: http://codereview.chromium.org/6628027

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