Fix release-iso makefile rule

Remove the dependency on cross from checksum.  This means that checksum
can be called without all binaries being built, but no checksums will
be generate for those binaries.

Makefile

@@ -127,7 +127,7 @@ $(GOPATH)/bin/go-bindata: $(GOPATH)/src/$(ORG)
 cross: out/localkube out/minikube-linux-amd64 out/minikube-darwin-amd64 out/minikube-windows-amd64.exe
 
 .PHONY: checksum
-checksum: cross
+checksum:
 	for f in out/localkube out/minikube-linux-amd64 out/minikube-darwin-amd64 out/minikube-windows-amd64.exe out/minikube.iso; do \
 		if [ -f "$${f}" ]; then \
 			openssl sha256 "$${f}" | awk '{print $$2}' > "$${f}.sha256" ; \