build_image: use string comparison for comparing strings
build_image: use string comparison for comparing strings

BUG=n0ne
TEST=./build_image and see it fail and ask me a question, instead of death

Change-Id: I27f7ac71ce74ff711e8b152ee202121d518045cd

Review URL: http://codereview.chromium.org/6417001

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