esp8266, stmhal, unix: MAKE_FROZEN is consistently defined in mkenv.mk.

esp8266/Makefile

@@ -12,7 +12,6 @@ include ../py/py.mk
 
 MPY_CROSS = ../mpy-cross/mpy-cross
 MPY_TOOL = ../tools/mpy-tool.py
-MAKE_FROZEN = ../tools/make-frozen.py
 
 FROZEN_DIR = scripts
 FROZEN_MPY_DIR = modules

stmhal/Makefile

@@ -276,7 +276,6 @@ all: $(BUILD)/firmware.dfu $(BUILD)/firmware.hex
 ifneq ($(FROZEN_DIR),)
 CFLAGS += -DMICROPY_MODULE_FROZEN_STR
 OBJ += $(BUILD)/frozen-files.o
-MAKE_FROZEN = ../tools/make-frozen.py
 
 $(BUILD)/frozen-files.o: $(BUILD)/frozen-files.c
 	$(call compile_c)

unix/Makefile

@@ -273,7 +273,7 @@ coverage_test: coverage
 	gcov -o build-coverage/extmod ../extmod/*.c
 
 $(BUILD)/_frozen_upip.c: $(BUILD)/frozen_upip/upip.py
-	../tools/make-frozen.py $(dir $^) > $@
+	$(MAKE_FROZEN) $(dir $^) > $@
 
 # Select latest upip version available
 UPIP_TARBALL := $(shell ls -1 -v ../tools/micropython-upip-*.tar.gz | tail -n1)