What's the "best" way to  fix the .config file generated by generate_config.pl ? it says it's using $KERNEL_CONFIG but that variable doesn't seem defined. so far I get
GOT: make: Leaving directory '/tmp/nix-build-linux-config-4.1.38-mptcp_v0.92.1.drv-0/mptcp-0.92.1'
warning: unused option: ARM_KPROBES_TEST
warning: unused option: B43_PCMCIA
warning: unused option: CPU_NOTIFIER_ERROR_INJECT
error: unused option: DRM_I915_KMS
builder for ‘/nix/store/fzkidgj6i9ia1v745wffm5dzl1vhmmlc-linux-config-4.1.38-mptcp_v0.92.1.drv’ failed with exit code 255