TFTP documentation

[irgendwie]

The PXE documentation in its current state isn't very helpful.

Since @privatereese is currently working on integrating a DHCP service into the Vagrant VMs, why not explain in this part of the documentation the boot flow we have for the real boards:

1. Build Fiasco.OC/Genode OS in Vagrant
2. Provide *.elf or *.img via TFTP
3. Boot platforms via U-Boot
4. Receive ip address, tftp server ip address, tftp image via DHCP
5. Boot the OS on the platforms

[privatereese]

Is already done in #23