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/userland/ring0.c

@@ -0,0 +1,14 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#ring0 */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "../ring0.h"
+
+int main(void) {
+#if defined(__x86_64__) || defined(__i386__)
+	Ring0Regs ring0_regs;
+	ring0_get_control_regs(&ring0_regs);
+#endif
+	return EXIT_SUCCESS;
+}