http://soekris.com/Manuals/net4801_manual.pdf

* set up infrastructure

The netboot image for debian is already around.  You need

wget http://centerclick.org/net4801/pxelinux/pxelinux.0.gz

as opposed to the pxelinux.bin that's already around.  You need to
make a link in pxelinux.cfg to an appropriate pxelinux configuration
file, namely the one for serial.  To the end of the 'insall' image you
want to add:

DEBIAN_FRONTEND=text

(we did this, so it's probably already there.)

Minicom should be set for 9600 baud, 8 databits, no parity, 1 stop
bit, no flow control.  Turn hardware flow control off in minicom.  The
UNIX device for talking to the console is almost certainly /dev/ttyS0.

* configure the Soekris net4801 BIOS

Look at the banner from when the net4801 boots.  Get the BIOS version
and check it against the Soekris web site.

Set the date and time.

set ConSpeed=9600