It's worth mentioning that you can't put everything in a module. Because modules are stored on disk, your bootable kernel image needs to have compiled-in support for your disk controller, drives, and your root filesystem. If you don't have these essential components compiled into your kernel image -- that is, if you try to compile them as modules instead -- then your kernel won't have the necessary ability to load these modules from disk, creating a rather ugly chicken-and-egg problem that will result in a kernel that can't boot your system!