For switches which remain permanently in the non-forwarding state (or
which erroneously report a non-forwarding state), ensure that iPXE
will eventually give up waiting for the link to become unblocked.

Originally-fixed-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

Relicence files with kind permission from the following contributors:

  Alex Williamson <alex.williamson@redhat.com>
  Eduardo Habkost <ehabkost@redhat.com>
  Greg Jednaszewski <jednaszewski@gmail.com>
  H. Peter Anvin <hpa@zytor.com>
  Marin Hannache <git@mareo.fr>
  Robin Smidsrød <robin@smidsrod.no>
  Shao Miller <sha0.miller@gmail.com>
  Thomas Horsten <thomas@horsten.com>

Signed-off-by: Michael Brown <mcb30@ipxe.org>

iPXE uses DHCP timeouts loosely based on values recommended by the
specification, but often abbreviated to reduce timeouts for reliable
and/or simple network topologies.  Extract the DHCP timing parameters
to config/dhcp.h and document them.  The resulting default iPXE
behavior is exactly the same, but downstreams are now afforded the
opportunity to implement spec-compliant behavior via config file
overrides.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>