Seperate source and work directories

Quite a few changes.

* Source files for the kernel and busybox and saved to a **source** folder. So they don't need to be downloaded every time you rebuild. To stop incomplete downloads, I've added -c to wget.
* Kernel source is cleaned with **make mrproper** instead of **make clean**
* Kernel is compiled with **make bzImage** instead of **make vmlinux**

I've also commented the code, explaining what everything does.