Fix typo breaking cros_run_unit_tests when default board isn't set.

FLAGS_board should only be relied upon after the arguments have been
parsed.

BUG=chromium-os:15853
TEST=Verified that emerge happens on board as it should instead of on
     host.

Change-Id: I40bd48ca769e3710d812bb50dec17346caefa92e
Reviewed-on: http://gerrit.chromium.org/gerrit/1806
Tested-by: David James <davidjames@chromium.org>
Reviewed-by: Jon Kliegman <kliegs@chromium.org>
1 file changed