Standardize invocations of emerge in build_image.
Standardize invocations of emerge in build_image.

Also includes a style nit fix:  don't quote arguments to numeric
comparison test operators.

BUG=chromium-os:13582
TEST=setup_board --force && build_packages && build_image; boot the result

Review URL: http://codereview.chromium.org/6801027

Change-Id: I689ca9697a05cf93a789fde1242fb2fa50f21612

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