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/timer.c

@@ -0,0 +1,39 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#timers */
+
+#include <linux/jiffies.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/timer.h>
+
+static int i;
+/* We would normally mark this as static and give it a more generic name.
+ * But let's do it like this this time for the sake of our GDB kernel module step debugging example. */
+void lkmc_timer_callback(struct timer_list *data);
+static unsigned long onesec;
+
+DEFINE_TIMER(mytimer, lkmc_timer_callback);
+
+void lkmc_timer_callback(struct timer_list *data)
+{
+	pr_info("%d\n", i);
+	i++;
+	if (i == 10)
+		i = 0;
+	mod_timer(&mytimer, jiffies + onesec);
+}
+
+static int myinit(void)
+{
+	onesec = msecs_to_jiffies(1000);
+	mod_timer(&mytimer, jiffies + onesec);
+	return 0;
+}
+
+static void myexit(void)
+{
+	del_timer(&mytimer);
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");