Tip: to build an linux-5.6 kernel, you can do:

(define linux-libre-5.6
  ((@@ (gnu packages linux) make-linux-libre*)
   (@@ (gnu packages linux) linux-libre-5.6-version)
   (@@ (gnu packages linux) linux-libre-5.6-source)
   '("x86_64-linux")))

(define-public my-linux
  (package
   (inherit linux-libre-5.6)
   (name "my-linux")
   (native-inputs
    `(("kconfig" ,(local-file "kernel-5.6.config"))
      ,@(alist-delete "kconfig"
                      (package-native-inputs linux))))))