build: create a local GOPATH if none specified

Instead of requiring the developer to set up their own GOPATH somewhere,
do like Kubernetes and OpenShift Origin do:

git clone xxxxx
cd xxxxx
make

by creating an _output/ directory and linking the local source tree
into it, and setting that to be the GOPATH.

Signed-off-by: Dan Williams <dcbw@redhat.com>

test/bin2img/Makefile

@@ -1,6 +0,0 @@
-bin2img: $(wildcard *.go)
-	go build -tags "$(BUILDTAGS)" -o $@
-
-.PHONY: clean
-clean:
-	rm -f bin2img