[Buildroot] How to handle custom kernel config

[Marco Trapanese]

Hello,
reading the docs I'm not sure how to handle a custom kernel config.
Let's say I have a custom linux.config file in a board/sub_directory.
In the make nconfig menu I selected the right path.

Now I want to change some settings. As far as I understand if I issue 
make linux-menuconfig it will read the default .config file, not the 
custom one. Am I wrong?

What's the right way to change those settings?
Thanks!

Marco