logging: Add knob to suppress PrintBuildbot* logging functions.

This CL adds a knob to modify the behavior of PrintBuildbot* functions.
By default, these functions now dump logs in a way that buidbot does not
recognize them as special log lines. When the knob is turned on, the
buildbot recognizable special log lines are dumped.

It also moves these functions to the cros_logging module, because they
belong better to the logging module.

Finally, by default, the cbuildbot entry point turns on this knob.

BUG=chromium:510905
TEST=(1) unittests.
     (2) Trybot the hell out of this.

Change-Id: I739dcbf13a60e20f68a1a692dba0c621ec297524
Reviewed-on: https://chromium-review.googlesource.com/286532
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index 226413a..b47475d 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -633,7 +633,7 @@
   else:
     sdk_version = options.sdk_version
   if options.buildbot_log_version:
-    cros_build_lib.PrintBuildbotStepText(sdk_version)
+    logging.PrintBuildbotStepText(sdk_version)
 
   # Based on selections, determine the tarball to fetch.
   if options.sdk_url: