add kernel nix builder

nix/bcachefs-kernel.nix

@@ -0,0 +1,34 @@
+{ lib
+, fetchpatch
+, fetchgit
+, fetchFromGitHub
+, buildLinux
+, commit
+, diffHash ? lib.fakeSha256
+, kernelVersion ? "5.13.0"
+, kernelPatches ? [] # must always be defined in bcachefs' all-packages.nix entry because it's also a top-level attribute supplied by callPackage
+, argsOverride ? {}
+, versionString ? (builtins.substring 0 8 commit)
+, ...
+} @ args:
+
+buildLinux {
+	inherit kernelPatches;
+
+	# pname = "linux";
+	version = "${kernelVersion}-bcachefs-${versionString}";
+	
+	modDirVersion = kernelVersion;
+	
+
+	src = fetchFromGitHub {
+		name = "bcachefs-kernel-src";
+		owner = "koverstreet";
+		repo = "bcachefs";
+		rev = commit;
+		sha256 = diffHash;
+	};
+
+	extraConfig = "BCACHEFS_FS m";
+	# NIX_DEBUG=5;
+}