py: Update makefiles to use $(CAT) variable instead of hard coded "cat".

The variable $(CAT) is initialised with the "cat" value in mkenv.mk like
for the other command line tools (rm, echo, cp, mkdir etc).  With this,
for example, Windows users can specify the path of cat.exe.

py/mkenv.mk

@@ -42,6 +42,7 @@ ECHO = @echo
 CP = cp
 MKDIR = mkdir
 SED = sed
+CAT = cat
 PYTHON = python3
 
 AS = $(CROSS_COMPILE)as

py/py.mk

@@ -338,7 +338,7 @@ MPCONFIGPORT_MK = $(wildcard mpconfigport.mk)
 # the lines in "" and then unwrap after the preprocessor is finished.
 $(HEADER_BUILD)/qstrdefs.generated.h: $(PY_QSTR_DEFS) $(QSTR_DEFS) $(QSTR_DEFS_COLLECTED) $(PY_SRC)/makeqstrdata.py mpconfigport.h $(MPCONFIGPORT_MK) $(PY_SRC)/mpconfig.h | $(HEADER_BUILD)
 	$(ECHO) "GEN $@"
-	$(Q)cat $(PY_QSTR_DEFS) $(QSTR_DEFS) $(QSTR_DEFS_COLLECTED) | $(SED) 's/^Q(.*)/"&"/' | $(CPP) $(CFLAGS) - | $(SED) 's/^"\(Q(.*)\)"/\1/' > $(HEADER_BUILD)/qstrdefs.preprocessed.h
+	$(Q)$(CAT) $(PY_QSTR_DEFS) $(QSTR_DEFS) $(QSTR_DEFS_COLLECTED) | $(SED) 's/^Q(.*)/"&"/' | $(CPP) $(CFLAGS) - | $(SED) 's/^\"\(Q(.*)\)\"/\1/' > $(HEADER_BUILD)/qstrdefs.preprocessed.h
 	$(Q)$(PYTHON) $(PY_SRC)/makeqstrdata.py $(HEADER_BUILD)/qstrdefs.preprocessed.h > $@
 
 # build a list of registered modules for py/objmodule.c.