refactor: move kernel functions to submodule

kernel/kernel_macos.go

@@ -0,0 +1,19 @@
+// Copyright 2023 Mikhail Klementev. All rights reserved.
+// Use of this source code is governed by a AGPLv3 license
+// (or later) that can be found in the LICENSE file.
+
+//go:build darwin
+// +build darwin
+
+package kernel
+
+import (
+	"errors"
+
+	"code.dumpstack.io/tools/out-of-tree/config"
+)
+
+func genHostKernels(download bool) (kcfg config.KernelConfig, err error) {
+	err = errors.New("generate host kernels for macOS is not supported")
+	return
+}