release: add sha256sum files (#1991)

Generate the sha256 sum of each asset. These should all be automatically
be uploaded in the github-push target.
Signed-off-by: Miek Gieben <miek@miek.nl>

Makefile.release

@@ -55,7 +55,7 @@ endif
 all:
 	@echo Use the 'release' target to build a release, 'docker' for docker build.
 
-release: pre build tar
+release: pre build tar sum
 
 docker: docker-build
 
@@ -86,6 +86,12 @@ tar:
 	    tar -zcf release/$(NAME)_$(VERSION)_linux_$$arch.tgz -C build/linux/$$arch $(NAME) ;\
 	done
 
+.PHONY: sum
+sum:
+	for asset in `ls -A release/*tgz`; do \
+	    sha256sum $$asset > $$asset.sha256; \
+	done
+
 .PHONY: github-push
 github-push:
 	@echo Releasing: $(VERSION)