Trying to set kernel build option L3_Master_Dev

I feel like I am missing something fundamental in how kernel options are set in OpenWrt.

In the past I have been able to set things like "CONFIG_KERNEL_CGROUPS=y" and when I build my target firmware in the kernel .config I see "CONFIG_CGROUPS=y". My understanding was that if you set CONFIG_KERNEL_XXXX it would be passed through to the kernel config as CONFIG_XXXX.

But I am having issues with getting the following to pass through. "CONFIG_KERNEL_NET_L3_MASTER_DEV=y" I see "# CONFIG_NET_L3_MASTER_DEV is not set" in the .config of the kernel. Clearly I can just set it in the .config, make clean (in the kernel) and rebuild the firmware and get the option into final image. But I don't know why this isn't working the way I thought.

I was hopeful that someone could point me in the right direction. Is there someplace that these "CONFIG_KERNEL" options are getting processed in the build and not just getting passed directly through to the .config?