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/strlen_overflow.c

@@ -0,0 +1,19 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#config_fortify_source */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/string.h>
+
+static int myinit(void)
+{
+	/* Missing terminaing NUL '\0'. */
+	char buf[] = {'p', 'w', 'n'};
+	pr_info("%llu\n", (long long unsigned)strlen(buf));
+	return 0;
+}
+
+static void myexit(void) {}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");