Default BuildBrokenDepfile to empty (false)

Test: The builds should still pass because all depfile is cleanup in
AOSP
Bug: 179452413

Change-Id: Ifb6f5320f72c3beb44ddd6654100645e0a62e659
diff --git a/core/board_config.mk b/core/board_config.mk
index 8074225..d280349 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -174,6 +174,7 @@
 
 
 _build_broken_var_list := \
+  BUILD_BROKEN_DEPFILE \
   BUILD_BROKEN_DUP_RULES \
   BUILD_BROKEN_DUP_SYSPROP \
   BUILD_BROKEN_ELF_PREBUILT_PRODUCT_COPY_FILES \