I couldn't find sufficient answers to my problem, even though it's probably very easy.
I have a headless NAS, which can just boot from USB. Right now freeNAS is running on it, because I just had to
dd the freenas-image onto the USB flash drive. It boots, gets it IP over DHCP and I can ssh into my machine.
How do I get the same functions (boot directly into the system without asking to install Ubuntu, DHCPclient and ssh started) with Ubuntu 12.04 server?