Add option to device's defconfig

I need to add specific option on the board's defconfig. I know where the defconfig file is located in the current version of cloned repository, and know how and from where to run make *.defconfig command to make the board configuration in effect.

But this means with each new version of clone I will have to locate u-boot stuff, which may reside in differently named directory, and perform manual changes to the file(s).

Question: what is the portable way - ideally triggered from the specific package - to add the config option into the specific defconfig and perform reconfiguration?

Thanks.

Edit: it is not possible to configure u-boot from the make menuconfig from the build root directory. only linux. To configure u-boot I must manually locate u-boot branch in build_dir and perform configuration from there.