[v2.7] cannot use CONFIG_COMPILER_WARNINGS_AS_ERRORS

[cfriedt]

Describe the bug
It would be nice to have a stable way of configuring whether warnings are promoted to errors in LTSv2.

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

• What target platform are you using? Any
• What have you tried to diagnose or workaround this issue? Looked at toolchain: Move extra warning options to toolchain abstraction #53305 and realized it was an easy backport
• Is this a regression? No

To Reproduce
Steps to reproduce the behavior:

1. git checkout origin/v2.7-branch
2. west build -p auto -t run samples/hello_world -- -DCONFIG_COMPILER_WARNINGS_AS_ERRORS=y
3. See (build) error due to undefined Kconfig option
4. add an unused variable in samples/hello_world/src/main.c
5. apply change in linked PR
6. see expected error when building (due to warning being promoted to error).

Expected behavior

• Build should succeed (there are no warnings in hello_world
• Warnings should be promoted to errors.

Impact
What impact does this issue have on your progress (e.g., annoyance, showstopper)

Logs and console output
Build error because Kconfig option is not defined

 warning: attempt to assign the value 'y' to the undefined symbol COMPILER_WARNINGS_AS_ERRORS

Expected error when promoting warnings to errors

error: unused variable 'foo' [-Werror=unused-variable]

Environment (please complete the following information):

• OS: Any
• Toolchain: Zephyr SDK v0.14.2
• Commit SHA: e677cfd

Additional context
Helpful for those migrating from LTSv2 to (pre) LTSv3

[stephanosio]

It would be nice to have a stable way of configuring whether warnings are promoted to errors in LTSv2.

That sounds more like an enhancement than a bug.

[cfriedt]

Fixed by #66501