Fix typo in build_image factory_test.
Fix typo in build_image factory_test.

The factory test image name is CHROMEOS_FACTORY_TEST_IMAGE_NAME.
Also fixed the error message when FLAGS_factory is not defined.

BUG=chromium-os:27651
TEST=build_image factory_test;
     # ls /usr/local/autotest/site_tests, seeing test packages.

Change-Id: Id8095e48e47382f07d77c85873bf588e489ba8cf
Reviewed-on: https://gerrit.chromium.org/gerrit/17816
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Ready: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

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