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/character_device.c

@@ -0,0 +1,43 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#character-devices */
+
+#include <linux/fs.h> /* register_chrdev, unregister_chrdev */
+#include <linux/module.h>
+#include <linux/seq_file.h> /* seq_read, seq_lseek, single_release */
+
+#define NAME "lkmc_character_device"
+
+static int major;
+
+static int show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "abcd");
+	return 0;
+}
+
+static int open(struct inode *inode, struct  file *file)
+{
+	return single_open(file, show, NULL);
+}
+
+static const struct file_operations fops = {
+	.llseek = seq_lseek,
+	.open = open,
+	.owner = THIS_MODULE,
+	.read = seq_read,
+	.release = single_release,
+};
+
+static int myinit(void)
+{
+	major = register_chrdev(0, NAME, &fops);
+	return 0;
+}
+
+static void myexit(void)
+{
+	unregister_chrdev(major, NAME);
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");