build_image: modify to not get devkeys from vboot_reference checkout
build_image: modify to not get devkeys from vboot_reference checkout

An earlier CL modifies the vboot_reference ebuild to install the
keys into /usr/share/vboot.

TEST=verified that build_image fails without this change and no
vboot_reference checkout but succeeds with this change and no
vboot_reference checkout

Change-Id: Ie2bc1091b52c36fd56cd5f763ee275afc91765c3

Review URL: http://codereview.chromium.org/2896008

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