speed up kernel build

by having two separate derivations for patching the kernel source tree
and building it, we have to copy said source trees from one store
location to another which takes non-neglible time on spinning rust
(literally minutes on my machine). Replace with a single derivation
that can do more things on one tree in-place

overlay.nix

@@ -12,6 +12,7 @@ final: prev: {
   s6-init-bin =  final.callPackage ./pkgs/s6-init-bin {};
   s6-rc-database = final.callPackage ./pkgs/s6-rc-database {};
 
+  kernel = final.callPackage ./pkgs/kernel {};
 
   dnsmasq =
     let d =  prev.dnsmasq.overrideAttrs(o: {