fs: new OCI filesystem type

fs: new OCI filesystem type

Add support to generate OCI (Open Container Initiative) images.

An OCI image consists of a manifest, an image index (optional), a set of
filesystem layers, and a configuration. The complete specification is
available in the link below:

https://github.com/opencontainers/image-spec/blob/master/spec.md

The image is generated with the host tool sloci-image, and config
options can be used to configure image parameters.

By default, the image is generated in a directory called rootfs-oci:

$ cd output/images
$ ls rootfs-oci/
blobs  index.json  oci-layout

Optionally, the image can be packed into a tar archive.

The image can be pushed to a registry using containers tools like
skopeo:

$ skopeo copy --dest-creds <user>:<pass> oci:rootfs-oci:<tag> \
	docker://<user>/<image>[:tag]

And then we can pull/run the container image with tools like docker:

$ docker run -it <user>/<image>[:tag]

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Matthew Weber <matthew.weber@collins.com>
[Arnout:
 - mention in help text that options are space separated;
 - use GO_GOARCH and GO_GOARM for architecture;
 - quote all arguments;
 - don't cd to BINARIES_DIR;
 - remove ROOTFS_OCI_IMAGE_NAME variable;
 - remove wildcard from rm.
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>