Some help on the build system

Dear rattling IoTlers,

we all know that RIOT's build system (using GNU make) is far from being easily understandable. Hence, I started to put some explanations into the Wiki. A first version can be found at https://github.com/RIOT-OS/RIOT/wiki/The-Make-Build-System

Please, feel free to extend and improve!

Cheers, Oleg