Default BINDDIR regardless of DOCKER_HOST

[Please be merciful as I am new to the Docker community]

I was [setting up the development environment](http://docs.docker.com/contributing/devenvironment/#using-your-built-docker-binary) when I noticed the binary was not available outside of the container - as I use boot2docker.

I am happy to use `BINDDIR=bundles make ...`, but I guess other new developers using boot2docker will be surprised so I wonder if the logic around `DOCKER_HOST` shall be there at all, or whether it shall be more complex (e.g. considering the presence of the `boot2docker` executable and the output of `boot2docker ip`). Hence this pull request.

I [signed off the commit](https://github.com/docker/docker/blob/5631ffbdfd252c369d393e41ee86008ab2d7eb7c/CONTRIBUTING.md#sign-your-work) even if this is a simple revert because otherwise CI fails.