linux: add passthru.isVanilla

For use by consumers that want to iterate over all Linux Kernel packages
to, e.g., find the latest “vanilla” version that matches some version
constraint. (E.g. This will soon be used by ZFS’s
latestCompatibleLinuxPackages)

pkgs/os-specific/linux/kernel/generic.nix

@@ -230,6 +230,7 @@ kernel.overrideAttrs (finalAttrs: previousAttrs: {
   passthru = previousAttrs.passthru or { } // basicArgs // {
     features = kernelFeatures;
     inherit commonStructuredConfig structuredExtraConfig extraMakeFlags isZen isHardened isLibre;
+    isVanilla = !(isHardened || isLibre || isZen);
     isXen = lib.warn "The isXen attribute is deprecated. All Nixpkgs kernels that support it now have Xen enabled." true;
 
     # Adds dependencies needed to edit the config: