py/mkenv.mk: Use $(TOP) instead of ".." to reference tools, mpy-cross.

py/mkenv.mk

@@ -58,9 +58,9 @@ CXX += -m32
 LD += -m32
 endif
 
-MAKE_FROZEN = ../tools/make-frozen.py
-MPY_CROSS = ../mpy-cross/mpy-cross
-MPY_TOOL = ../tools/mpy-tool.py
+MAKE_FROZEN = $(TOP)/tools/make-frozen.py
+MPY_CROSS = $(TOP)/mpy-cross/mpy-cross
+MPY_TOOL = $(TOP)/tools/mpy-tool.py
 
 all:
 .PHONY: all