Update build instruction for BACNet in README

in r3189 Steve added new feature to bacnet-stack: network port - to
access configuration and properties of network ports of a device.
https://sourceforge.net/p/bacnet/code/3189/

It's not implemented by Beremiz.

from __future__ import absolute_import

from etherlab.etherlab import *
from util.BitmapLibrary import AddBitmapFolder
import util.paths as paths

AddBitmapFolder(os.path.join(paths.AbsDir(__file__), "images"))