linux: Add 4.15

pkgs/os-specific/linux/kernel/linux-4.15.nix

···
+{ stdenv, buildPackages, hostPlatform, fetchurl, perl, buildLinux, ... } @ args:
+
+with stdenv.lib;
+
+import ./generic.nix (args // rec {
+  version = "4.15";
+
+  # modDirVersion needs to be x.y.z, will automatically add .0 if needed
+  modDirVersion = concatStrings (intersperse "." (take 3 (splitString "." "${version}.0")));
+
+  # branchVersion needs to be x.y
+  extraMeta.branch = concatStrings (intersperse "." (take 2 (splitString "." version)));
+
+  src = fetchurl {
+    url = "mirror://kernel/linux/kernel/v4.x/linux-${version}.tar.xz";
+    sha256 = "0sd7l9n9h7vf9c6gd6ciji28hawda60yj0llh17my06m0s4lf9js";
+  };
+} // (args.argsOverride or {}))

pkgs/top-level/all-packages.nix

···
     kernelPatches = with kernelPatches; [
       kernelPatches.bridge_stp_helper
       kernelPatches.modinst_arg_list_too_long
-      kernelPatches.cpu-cgroup-v2."4.11"
       kernelPatches.tag_hardened
     ];
     extraConfig = import ../os-specific/linux/kernel/hardened-config.nix {
···
         # See pkgs/os-specific/linux/kernel/cpu-cgroup-v2-patches/README.md
         # when adding a new linux version
         kernelPatches.cpu-cgroup-v2."4.11"
+        kernelPatches.modinst_arg_list_too_long
+      ]
+      ++ lib.optionals ((platform.kernelArch or null) == "mips")
+      [ kernelPatches.mips_fpureg_emu
+        kernelPatches.mips_fpu_sigill
+        kernelPatches.mips_ext3_n32
+      ];
+  };
+
+  linux_4_15 = callPackage ../os-specific/linux/kernel/linux-4.15.nix {
+    kernelPatches =
+      [ kernelPatches.bridge_stp_helper
+        # See pkgs/os-specific/linux/kernel/cpu-cgroup-v2-patches/README.md
+        # when adding a new linux version
+        # kernelPatches.cpu-cgroup-v2."4.11"
         kernelPatches.modinst_arg_list_too_long
       ]
       ++ lib.optionals ((platform.kernelArch or null) == "mips")
···
   linux = linuxPackages.kernel;
 
   # Update this when adding the newest kernel major version!
-  linuxPackages_latest = linuxPackages_4_14;
+  linuxPackages_latest = linuxPackages_4_15;
   linux_latest = linuxPackages_latest.kernel;
 
   # Build the kernel modules for the some of the kernels.