package/docker-engine: split docker-{cli, engine}, bump to v18.09.0

package/docker-engine: split docker-{cli, engine}, bump to v18.09.0

Docker upstream has split the Docker daemon and CLI into separate
codebases:

 - github.com/docker/engine: daemon, "dockerd" binary
 - github.com/docker/cli: "docker" command line interface

This commit splits the docker-engine package into docker-engine and
docker-cli.  Conveniently, the Docker project has begun maintaining
two separate release-tagged repositories for the CLI and daemon as of
v18.06-ce-rc1. Previous versions were tagged in a common "docker-ce"
repository which makes compilation awkward for Buildroot, especially
due to some limitations in the new Go package infrastructure.

Docker repositories "engine" and "cli" recently started tagging
releases. Select the latest stable release, v18.09.0.

The CLI is no longer automatically included with the engine. Users
will need to select BR2_PACKAGE_DOCKER_CLI to produce a both docker
and dockerd target binaries.

Docker CLI can be statically compiled. This enables usage of the
system docker client binary to access the parent daemon API from
within containers, where shared libraries are not available.

While at it, drop the useless host-go dependency from docker-engine,
since it's already added by the golang-package infrastructure.

Signed-off-by: Christian Stewart <christian@paral.in>
[Thomas: drop the host-go dependency from both docker-cli and
docker-engine]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>