Makefile: error message on make program/flash

Makefile

@@ -1,5 +1,5 @@
-SUBDIRS = tools rv32 fpga
-
+SUBDIRS   := tools rv32 fpga
+REVISIONS := v1 v2
 
 all clean spotless :
 	$(MAKE) local.$@ $(SUBDIRS) goal=$@
@@ -7,7 +7,7 @@ all clean spotless :
 prefpga:
 	$(MAKE) local.all $(filter-out fpga,$(SUBDIRS)) goal=all
 
-v1 v2 : prefpga
+$(REVISIONS): prefpga
 	$(MAKE) -C fpga $@
 
 .PHONY: $(SUBDIRS)
@@ -25,6 +25,11 @@ local.clean:
 
 local.spotless: local.clean
 
+# Obsolete target, need version
+program flash:
+	@echo 'Please specify one of: $(foreach rev,$(REVISIONS),$@-$(rev))' 1>&2
+	@false
+
 # FPGA-specific targets
 program-% flash-%: prefpga
 	$(MAKE) -C fpga $@