buildroot/docs/manual/customize-kernel-config.txt

Customizing the Linux kernel configuration
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The Linux kernel configuration can be customized just like
xref:busybox-custom[BusyBox] and xref:uclibc-custom[uClibc] using
+make linux-menuconfig+. Make sure you have enabled the kernel build
in +make menuconfig+ first. Once done, run +make+ to (re)build
everything.

If you want to use an existing config file for Linux, then see
xref:env-vars[].