py: Move frozen bytecode Makefile rules from ports to common mk files.

Now, to use frozen bytecode all a port needs to do is define
FROZEN_MPY_DIR to the directory containing the .py files to freeze, and
define MICROPY_MODULE_FROZEN_MPY and MICROPY_QSTR_EXTRA_POOL.

py/mkenv.mk

@@ -59,6 +59,8 @@ LD += -m32
 endif
 
 MAKE_FROZEN = ../tools/make-frozen.py
+MPY_CROSS = ../mpy-cross/mpy-cross
+MPY_TOOL = ../tools/mpy-tool.py
 
 all:
 .PHONY: all