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/hello.c

@@ -0,0 +1,22 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#getting-started-natively */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+static int myinit(void)
+{
+	pr_info("hello init 2\n");
+	/* 0 for success, any negative value means failure,
+	 * E* consts if you want to specify failure cause.
+	 * https://www.linux.com/learn/kernel-newbie-corner-loadable-kernel-modules-coming-and-going */
+	return 0;
+}
+
+static void myexit(void)
+{
+	pr_info("hello exit\n");
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");