add top_dir in the correct place

build_keyboard.mk

@@ -12,6 +12,8 @@ endif
 
 .DEFAULT_GOAL := all
 
+TOP_DIR ?= .
+
 include $(TOP_DIR)/common.mk
 
 # Set the filename for the final firmware binary

common.mk

@@ -1,6 +1,4 @@
 # Directory common source files exist
-TOP_DIR ?= .
-
 include $(TOP_DIR)/message.mk
 
 TMK_DIR = tmk_core