Deprecate serve_only for archive_dir.

Also change when the devserver will attempt to generate a payload, and
when it will attempt to generate a payload from the latest local build.

Previously, generation was attempted when no --image or --payload was
specified, and if archive_dir's serve_only mode was off.

Since serve_only is gone, we now just check that we're within chroot
before attempting to generate a payload.

Before attempting to generate a payload from the most recent local
build, we check that the directory specified by the label (if provided)
doesn't contain an update file.

BUG=chromium:261775
TEST=unittests updated
cbuildbot --remote mario-paladin

Change-Id: If094db197005836d9fcad5fb63ffcb8470be3706
Reviewed-on: https://gerrit.chromium.org/gerrit/63363
Reviewed-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Joy Chen <joychen@chromium.org>
Tested-by: Joy Chen <joychen@chromium.org>
Commit-Queue: Joy Chen <joychen@chromium.org>
diff --git a/build_artifact.py b/build_artifact.py
index c776def..2934467 100644
--- a/build_artifact.py
+++ b/build_artifact.py
@@ -185,7 +185,7 @@
     # Rename to update.gz.
     install_path = os.path.join(self.install_dir, self.name)
     new_install_path = os.path.join(self.install_dir,
-                                    devserver_constants.ROOT_UPDATE_FILE)
+                                    devserver_constants.UPDATE_FILE)
     shutil.move(install_path, new_install_path)