Added ability to download images othert than x86-generic.
Added ability to download images othert than x86-generic.

BUG=chromium-os:24907
TEST=make sure ./bin/cros_download_latest_image downloads the correct board

Change-Id: I8c1784edf39a6a04d46bc8cdbf7b62d2a361d593
Reviewed-on: https://gerrit.chromium.org/gerrit/13948
Reviewed-by: Daniel Erat <69d56254d3d393520d06e9838c23c147a5fee7d9@chromium.org>
Tested-by: Zelidrag Hornung <2423ba2b026624ad482ad8e6887b2d657f9c0c43@chromium.org>

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