cros_build_lib.IsInsideChroot: unify Die checking

A lot of places use IsInsideChroot to assert the code is being run in or
out side of the chroot.  Unify that logic in IsInsideChroot via a new arg.

BUG=None
TEST=buildbot/run_tests passed
TEST=`cros_sdk -- true` works but `cros_sdk -- cros_sdk` fails

Change-Id: If600dc96e15fabdab7a8f96cdc7f5551a00b3837
Reviewed-on: https://gerrit.chromium.org/gerrit/39149
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 4911be7..2f0137a 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1097,9 +1097,7 @@
 
   _PostParseCheck(parser, options, args)
 
-  if cros_build_lib.IsInsideChroot():
-    cros_build_lib.Die('Please run cbuildbot from outside the chroot.')
-
+  cros_build_lib.AssertOutsideChroot()
 
   if options.remote:
     cros_build_lib.logger.setLevel(logging.WARNING)