Kconfig: make app.config and app.config.test mutually exclusive

[miri64]

Contribution description

When an application contains an app.config for configuration using Kconfig without dependency resolution and an app.config.test for configuration using Kconfig with dependency resolution, the app.config file is still loaded, setting legacy symbols, and this is causing errors with Kconfig (see e.g. #15951 (comment)). This makes app.config and app.config.test mutually exclusive and their selection as application base configuration based on the setting of TEST_KCONFIG=1.

Testing procedure

I cherry-picked this PR on top of #15951 and try to build the test there with and without TEST_KCONFIG=1 (currently causing an easy to solve merge conflict

$ git remote add miri64 git@github.com:miri64/RIOT.git
$ git fetch miri64
$ git checkout miri64/congure/feat/initial
$ git cherry-pick miri64/kconfig/fix/app.config-mutual-exclusive
$ TEST_KCONFIG=1 make -C tests/congure_test -j
$ TEST_KCONFIG=1 make -C tests/congure_test -j clean   # otherwise one get's errors
$ make -C tests/congure_test -j
<!--
Details steps to test your contribution:
- which test/example to compile for which board and is there a 'test' command
- how to know that it was not working/available in master
- the expected success test output
-->


### Issues/PRs references

<!--
Examples: Fixes #1234. See also #5678. Depends on PR #9876.

Please use keywords (e.g., fixes, resolve) with the links to the issues you
resolved, this way they will be automatically closed when your pull request
is merged. See https://help.github.com/articles/closing-issues-using-keywords/.
-->

[MrKevinWeiss]

As long as people know that they need to double the efforts until the migration is complete that is OK with me but I will let @leandrolanzieri have the final word.

[MrKevinWeiss]

I guess nobody opposes the idea and the implementation seems sound so ACK!

[MrKevinWeiss]

Lets holdoff on the merge for a second. Do other app.config parameter get used with app.config.test?

[MrKevinWeiss]

It looks like app.config only exists in some tests/gnrc_* tests/kconfig

None of which contain a app.config.test so I guess we are OK...

[miri64]

Thanks for the review!