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: Ibc7b140691b5eaf5878994e041ac01d827099cf4
Previously-Reviewed-on: https://gerrit.chromium.org/gerrit/33266
Reviewed-on: https://chromium-review.googlesource.com/194423
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed