The port number should default to "0" and bitbake let the kernel pick a free port number, which bitbake will then pass back to the env script for setting the environment.
Jason has patches out for this on the bitbake list
Fixed in master by Jason: http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=6ff9c9e39de57964ad7f711a77243a7049e9cada