terraform/oci: Generate Kconfig.images dynamically

Duplicate the same functionality as is already provided for AWS.

The generated image menu includes platform images that are available
in the region or regions that the user tenancy is subscribed to.

Users can still manually select marketplace images by selecting the
"Custom image OCID" option and pasting in the image's OCID directly.

Generated-by: Claude AI
Signed-off-by: Chuck Lever <[email protected]>

Author: chucklever