(we really don't need to support multiple interfaces)
Also arrange things so that we are able to unload the PXE stacks only
when we are sure that we don't need them anymore. (To make this more
useful, a hook in exec() is needed.
written by Alfred Perlstein, Paul Saab, John Baldwin for FreeBSD.
Still needs some work, but this will load a kernel on a Soekris net4501.
Many thanks to Frank van der Linden and his simulator wizardry for
helping me debug the low-level startup code.
