Fixed formatting in build_image $VAR -> ${VAR}.
Fixed formatting in build_image $VAR -> ${VAR}.

Also remove trailing spaces, and add punctuation to comments.
TEST=Ran build_image
BUG=None

Review URL: http://codereview.chromium.org/2804012

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