doc: Detailed example for netconsole setup

This adds details that I would have liked to have readily available,
in particular how to activate the network interface before enabling
netconsole, and how to integrate netconsole so you can use the U-Boot
prompt.

Signed-off-by: Fiona Klute <fiona.klute@gmx.de>
Reviewed-by: Tony Dinh <mibodhi@gmail.com>