Security Report: Perform hardening on Zephyr apps before MESH release

[adamfowleruk]

Zephyr includes tools to help with checking the hardening configuration of apps. Principally:-
https://docs.zephyrproject.org/latest/security/hardening-tool.html

Run these before release for all end-user intended apps.

Also ensure our secure updates and OTA are working as per secure design principles:-
https://docs.zephyrproject.org/latest/security/security-overview.html#system-level-security-ecosystem

Also document the threat model for each final app, as well as each Herald module (E.g. Core, Mesh, Modem etc.).