[v2.7]: subsys: coredump: unable to define out-of-tree backend

[cfriedt]

Describe the bug
The backend concept is part of Coredump to support some flexibility for API consumers. Currently, there is a distinct lack of flexibility, because either CONFIG_DEBUG_COREDUMP_BACKEND_LOGGING or CONFIG_DEBUG_COREDUMP_BACKEND_FLASH_PARTITION must be used.

Note: other subsystems work around the issue of requiring direct, named linkage by using iterable sections.

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using? Irrelevant
• What have you tried to diagnose or workaround this issue? looked at what was done in main
• Is this a regression? If yes, have you been able to "git bisect" it to a
  specific commit? No

To Reproduce
Steps to reproduce the behavior:

1. define a custom coredump backend with LTSv2
2. observe compile failure

Expected behavior
Be able to define a custom coredump backend for use in LTSv2

Impact
showstopper

Logs and console output

error: Need to select a coredump backend

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows): irrelevant
• Toolchain (e.g Zephyr SDK, ...): irrelevant
• Commit SHA or Version used: f0d2a3e

Additional context
The PR for main that fixed this issue is #41663