Support running with multiple boards at once.

The --board option now accepts a list of colon-separated board names
and will run over all the boards in one run.  This applies to both
the "status report" and "upgrade" modes.

BUG=chromium-os:17353
TEST=run cros_portage_upgrade with --board=x86-mario:tegra2_kaen
or anything similar

Change-Id: Ib080c56d68e26b3ef52365f100134b1c55ad0c52
Reviewed-on: http://gerrit.chromium.org/gerrit/4217
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: Matt Tennant <mtennant@chromium.org>
7 files changed