kbuild: Remove all .*.cmd files on make mrproper

We skip removing scripts/lxdialog/.*.cmd on make clean, which is
on purpose since we want lxdialog to survive here. But on
make mrproper these should go as well.

Makefile

@@ -607,7 +607,8 @@ clean:	archclean
 
 mrproper: clean archmrproper
 	@echo 'Making mrproper'
-	@find . \( -size 0 -o -name .depend \) -type f -print | xargs rm -f
+	@find . \( -size 0 -o -name .depend -o -name .\*.cmd \) \
+		   -type f -print | xargs rm -f
 	@rm -f $(MRPROPER_FILES)
 	@rm -rf $(MRPROPER_DIRS)
 	@$(MAKE) -C Documentation/DocBook mrproper