py: Update makefiles to use $(TOUCH) instead of hard coded "touch".

The variable $(TOUCH) is initialized with the "touch" 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 touch.exe.

py/mkenv.mk

@@ -43,6 +43,7 @@ CP = cp
 MKDIR = mkdir
 SED = sed
 CAT = cat
+TOUCH = touch
 PYTHON = python3
 
 AS = $(CROSS_COMPILE)as

py/mkrules.mk

@@ -77,7 +77,7 @@ $(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(HEADER_
 $(HEADER_BUILD)/qstr.split: $(HEADER_BUILD)/qstr.i.last
 	$(ECHO) "GEN $@"
 	$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py split $(HEADER_BUILD)/qstr.i.last $(HEADER_BUILD)/qstr $(QSTR_DEFS_COLLECTED)
-	$(Q)touch $@
+	$(Q)$(TOUCH) $@
 
 $(QSTR_DEFS_COLLECTED): $(HEADER_BUILD)/qstr.split
 	$(ECHO) "GEN $@"
@@ -190,7 +190,7 @@ print-cfg:
 
 print-def:
 	@$(ECHO) "The following defines are built into the $(CC) compiler"
-	touch __empty__.c
+	$(TOUCH) __empty__.c
 	@$(CC) -E -Wp,-dM __empty__.c
 	@$(RM) -f __empty__.c