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/myprintk.c

@@ -0,0 +1,22 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#pr_debug */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+static int myinit(void)
+{
+	pr_alert("pr_alert\n");
+	pr_crit("pr_crit\n");
+	pr_err("pr_err");
+	pr_warning("pr_warning\n");
+	pr_notice("pr_notice\n");
+	pr_info("pr_info\n");
+	pr_debug("pr_debug\n");
+	return 0;
+}
+
+static void myexit(void) { }
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");