bring a minimal buildroot kernel modules example to life

More importantly, to make modules_install on the Linux kernel build.

buildroot_packages/kernel_modules/buildroot_hello.c

@@ -0,0 +1,19 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#getting-started-natively */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+static int myinit(void)
+{
+	pr_info("init buildroot\n");
+	return 0;
+}
+
+static void myexit(void)
+{
+	pr_info("exit buildroot\n");
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");