config_lib: Add 'debug' build config value.

This (so far unused) config value will force builds to run as if
--debug had been set on the command line.

BUG=chromium:776955
TEST=run_tests

Change-Id: I07bfef39bdc46b54dfb72d41ef47e0f9de1f9ac0
Reviewed-on: https://chromium-review.googlesource.com/731475
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index f337ae1..6fd5b79 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -702,6 +702,11 @@
     cros_build_lib.Die('payload configs (%s) require --channel to do anything'
                        ' useful.' % options.build_config_name)
 
+  # If the build config explicitly forces the debug flag, set the debug flag
+  # as if it was set from the command line.
+  if build_config.debug:
+    options.debug = True
+
   # The --version option is not compatible with an external target unless the
   # --buildbot option is specified.  More correctly, only "paladin versions"
   # will work with external targets, and those are only used with --buildbot.