[rom_ext] Add an update_policy to the ownership block, allowing the owner to specify how ownership updates should be performed

Examples:
- Full unlock allowed.
- Unlock to Self only.
- Accept signed updates from Self if config_version is greater.

The issue identifies the rom_ext ownership block but no files or tests. Start by locating that block and its existing update handling, then clarify how the three listed policies should behave and how policy configuration is represented. Done means the ownership block supports the agreed update policies with coverage for each example.

Written by the indexing model from the issue text.