Makefile: Use APP_CUSTOM_BUILD_DEPENDENCIES instead of DEP_DIR

Makefile

@@ -185,18 +185,14 @@ load: all
 delete:
 	python3 -m ledgerblue.deleteApp $(COMMON_DELETE_PARAMS)
 
-# import generic rules from the sdk
-include $(BOLOS_SDK)/Makefile.rules
-
 listvariants:
 	@echo VARIANTS APP tezos_wallet tezos_baking
 
-# Define DEP_DIR to keep compatibility with old SDK
-ifeq ($(DEP_DIR),)
-	DEP_DIR := dep
-endif
-
 # Generate delegates from baker list
 src/delegates.h: tools/gen-delegates.sh tools/BakersRegistryCoreUnfilteredData.json
 	bash ./tools/gen-delegates.sh ./tools/BakersRegistryCoreUnfilteredData.json
-$(DEP_DIR)/to_string.d $(DEP_DIR)/app/src/to_string.d: src/delegates.h
+
+APP_CUSTOM_BUILD_DEPENDENCIES = src/delegates.h
+
+# import generic rules from the sdk
+include $(BOLOS_SDK)/Makefile.rules