Add help target and ability to generate specific models

Makefile

@@ -9,6 +9,19 @@ CONFIG = Lego.json
 .PHONY: all
 all: models thumbnails
 
+.PHONY: help
+help:
+	@echo Targets:
+	@echo all          Builds models and thumbnails
+	@echo help         Shows this help file
+	@echo clean        Removes generated files
+	@echo models       Builds all models
+	@echo thumbnails   Generates model thumbnails
+	@echo webpage      Generates web page
+	@echo MODEL        Builds an STL and a PNG for the given model
+	@echo
+	@echo Available models: $(PARAMETERS)
+
 .PHONY: clean
 clean:
 	rm -f $(STL) $(PNG) $(INDEX)
@@ -22,6 +35,8 @@ thumbnails: $(PNG)
 .PHONY: webpage
 webpage: thumbnails $(INDEX)
 
+$(PARAMETERS): % : $(DESTDIR)/%.png $(DESTDIR)/%.stl
+
 $(STL) $(PNG): $(SCAD) $(CONFIG) Makefile
 	mkdir -p $(@D)
 	openscad -o $@ -p $(CONFIG) -P $(basename $(@F)) --camera=0,0,0,60,0,135,0 --autocenter --viewall $(SCAD)