Multiple profiles create artifacts, these should be stored in the JSON
file as well, allowing downstream tooling to show those files, too.

Artifacts don't have specific filesystems so only the fields `name`,
`type` and `sha256` are available.

Rename env variable names from IMAGE_ to FILE_ prefixes to reflect that
images, kernels and artifacts are added with the same command.

Signed-off-by: Paul Spooren <mail@aparcar.org>