Skip chroot upgrades when bootstrapping.
Skip chroot upgrades when bootstrapping.

If we're bootstrapping, there's no need to ever update the chroot in
SetupBoardStage. So skip it.

This optimization also coincidentally avoids some faulty logic in
cros_setup_toolchains, which is helpful too.

BUG=none
TEST=chromiumos-sdk with --latest-toolchain

Change-Id: Id9b228a98094edf182f497ab2528969411564a46
Reviewed-on: https://gerrit.chromium.org/gerrit/46332
Reviewed-by: Han Shen <025253d3aa95f32ae4a9bb41f0f6bed72b33d2ce@google.com>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite