crosutils: Always pass an image to devserver so it can pregenerate
crosutils: Always pass an image to devserver so it can pregenerate

Change-Id: Ib366064df269bd0d4aee30e5b1628938bd8fedf9

BUG=8313
TEST=Running it once without passing an image

Review URL: http://codereview.chromium.org/4208003

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