modules: rename packages/kernel_modules to packages/lkmc

Then inside, split packages/lkmc into kernel_modules and userland,
to keep userland out of the kernel_modules parent path, which makes no
sense.

Copy built modules and userland to the output rootfs overlay.

Document Linux distro tradeoffs.

packages/lkmc/kernel_modules/kthread.c

@@ -0,0 +1,38 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#kthread */
+
+#include <linux/delay.h> /* usleep_range */
+#include <linux/kernel.h>
+#include <linux/kthread.h>
+#include <linux/module.h>
+
+static struct task_struct *kthread;
+
+static int work_func(void *data)
+{
+	u32 i = 0;
+	while (!kthread_should_stop()) {
+		pr_info("%u\n", i);
+		usleep_range(1000000, 1000001);
+		i++;
+		if (i == 10)
+			i = 0;
+	}
+	return 0;
+}
+
+static int myinit(void)
+{
+	kthread = kthread_create(work_func, NULL, "mykthread");
+	wake_up_process(kthread);
+	return 0;
+}
+
+static void myexit(void)
+{
+	/* Waits for thread to return. */
+	kthread_stop(kthread);
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");