Improve checks for include dirs outside the source tree

See the Changes.md addition for more details.

Test: treehugger (build_test on downstream branches)
Change-Id: I6ad6b454a6fccf93772fda26cfc7dd6bbfc07c40
diff --git a/core/board_config.mk b/core/board_config.mk
index 4c128f1..0e3c52f 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -89,6 +89,7 @@
   BUILD_BROKEN_PREBUILT_ELF_FILES \
   BUILD_BROKEN_TREBLE_SYSPROP_NEVERALLOW \
   BUILD_BROKEN_USES_NETWORK \
+  BUILD_BROKEN_OUTSIDE_INCLUDE_DIRS \
 
 _build_broken_var_list += \
   $(foreach m,$(AVAILABLE_BUILD_MODULE_TYPES) \