setup_board: quote flags var
setup_board: quote flags var

Now that the flags variable can be "[stable]", if we don't quote its
expansion, it can match files like "t" resulting in bad flags being
passed to crossdev.

BUG=None
TEST=`touch t; ./setup_board --board=x86-alex` no longer results in "tgcc"

Change-Id: I866e934776e2d47a2c99bd08964017014aace97f
Reviewed-on: https://gerrit.chromium.org/gerrit/19595
Reviewed-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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