build_packages: allow people to specify a specific set of packages
build_packages: allow people to specify a specific set of packages

This way we can tweak buildbot configs to install specific packages rather
than all of ChromeOS (useful for firmware builders).

BUG=chromium-os:19140
TEST=`cbuildbot {{x86,amd64}-generic,daisy}-paladin {amd64,arm,x86}-generic-full stout-release` passed
TEST=`./build_packages nano` installed just nano

Change-Id: I9c316d7c390ebe6ceade97cf33c1387dc83fb887
Reviewed-on: https://gerrit.chromium.org/gerrit/33266
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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