Load keymap-level rules.mk when compiling keymap.json (#8841)

build_json.mk

@@ -21,6 +21,11 @@ else ifneq ("$(wildcard $(MAIN_KEYMAP_PATH_1)/keymap.json)","")
     KEYMAP_PATH := $(MAIN_KEYMAP_PATH_1)
 endif
 
+# Load the keymap-level rules.mk if exists
+ifneq ("$(wildcard $(KEYMAP_PATH))", "")
+    -include $(KEYMAP_PATH)/rules.mk
+endif
+
 # Generate the keymap.c
 $(KEYBOARD_OUTPUT)/src/keymap.c: $(KEYMAP_JSON)
 	bin/qmk json2c --quiet --output $(KEYMAP_C) $(KEYMAP_JSON)