mpy-cross/Makefile: Override undefine any inherited variables.

This is a temporary hack to allow othe Makefiles to build mpy-cross
recursively.

mpy-cross/Makefile

+# The following is a temporary hack to forefully undefine vars that might have
+# be defined by a calling Makefile (from recursive make).
+# TODO: Find a better way to be able to call this Makefile recursively.
+override undefine COPT
+override undefine CFLAGS_EXTRA
+override undefine LDFLAGS_EXTRA
+override undefine FROZEN_DIR
+override undefine FROZEN_MPY_DIR
+override undefine BUILD
+override undefine PROG
+
 include ../py/mkenv.mk
 
 # define main target