kbuild: teach kselftest-merge to find nested config files

Current implementation of kselftest-merge only finds config files that
are one level deep using `$(srctree)/tools/testing/selftests/*/config`.

Often, config files are added in nested directories, and do not get
picked up by kselftest-merge.

Use `find` to catch all config files under
`$(srctree)/tools/testing/selftests` instead.

Signed-off-by: Dan Rue <dan.rue@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>