fix typo in Makefile GITREV release target

Makefile

@@ -77,7 +77,7 @@ release : clean ui assets
 	mkdir $(BUILDPATH)/keys
 	$(MAKE) build-linux
 	mv build/$(BINARY_NAME)-linux-amd64 $(BUILDPATH)/$(BINARY_NAME)
-	tar -cvzf $(BINARY_NAME)_$(GIT_REV)_linux_amd64.tar.gz -C build $(BINARY_NAME)
+	tar -cvzf $(BINARY_NAME)_$(GITREV)_linux_amd64.tar.gz -C build $(BINARY_NAME)
 	rm $(BUILDPATH)/$(BINARY_NAME)
 	$(MAKE) build-arm7
 	mv build/$(BINARY_NAME)-linux-arm-7 $(BUILDPATH)/$(BINARY_NAME)