setup_board: add a skip_chroot_upgrade flag
setup_board: add a skip_chroot_upgrade flag

We need this if we want to call setup_board from a upgrade script.

BUG=chrome-os-partner:8235
TEST=add new upgrade script, run setup_board --skip_version_hooks
ensure new upgrade script isn't run

Change-Id: I31341d0b9a5c378a8288ab42f8506db90c2299fa
Reviewed-on: https://gerrit.chromium.org/gerrit/16941
Commit-Ready: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Reviewed-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Tested-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>

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