selftests: netdevsim: add a config file

netdevsim tests aren't very well integrated with kselftest,
which has its advantages and disadvantages. But regardless
of the intended integration - a config file to know what kernel
to build is very useful, add one.

Fixes: fc4c93f1 ("selftests: add basic netdevsim devlink flash testing")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20240116154311.1945801-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>