Add instructions for tftp booting

[mithro]

LiteX supports loading the firmware via tftp.

Would be nice to have some instructions to get that working. Should also mean you can support the device with the smaller spi flash.

Can probably steal some instructions from https://github.com/timvideos/litex-buildenv/wiki/HowTo-LCA2018-FPGA-Miniconf-VexRiscv-Renode

[skiphansen]

Absolutely. Will be done before I release any builds for the Rev C device.