small update, allow CONFIG_MK to be pre-defined before inclusion
diff --git a/builds/toplevel.mk b/builds/toplevel.mk
index d353b4c..c9f9cc6 100644
--- a/builds/toplevel.mk
+++ b/builds/toplevel.mk
@@ -28,7 +28,9 @@
.PHONY: setup
+ifndef CONFIG_MK
CONFIG_MK := config.mk
+endif
# If no configuration sub-makefile is present, or if `setup' is the target
# to be built, run the auto-detection rules to figure out which