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/init_module.c

@@ -0,0 +1,16 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#init_module */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+int init_module(void)
+{
+	pr_info("init_module\n");
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	pr_info("cleanup_module\n");
+}
+MODULE_LICENSE("GPL");