Add more BUILD_BROKEN_* logic

Make the values tristate, so that we can differentiate set vs not in
scripts that parse the logs.

Also add some placeholders for warnings that I expect to turn into
errors.

Test: Set BUILD_BROKEN_PHONY_TARGETS to "foo"
Change-Id: I3f7e1e0f85dcef84d6461fc40bdd83dc5bf75ff7
diff --git a/core/config.mk b/core/config.mk
index 0920b56..9f8426c 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -208,8 +208,10 @@
 # ###############################################################
 # Broken build defaults
 # ###############################################################
-# Assume that all boards have duplicate rules right now.
-BUILD_BROKEN_DUP_RULES := true
+BUILD_BROKEN_ANDROIDMK_EXPORTS :=
+BUILD_BROKEN_DUP_COPY_HEADERS :=
+BUILD_BROKEN_DUP_RULES :=
+BUILD_BROKEN_PHONY_TARGETS :=
 
 # ###############################################################
 # Include sub-configuration files