release: Publish OCI images on "ftp" mirrors

If WITH_OCIIMAGES is set, "make ftp-stage" will now publish those bits
into a directory under /OCI-IMAGES/, similar to how we publish CI and
VM images.

Approved by: re (cperciva)
MFC after: 1 minute

(cherry picked from commit e95e3e6eb7ef723aa5dd08de48a9ad3fec647c7d)
(cherry picked from commit e8c5d6f6b89f38ed7c20ad5537fc82092106d378)