[netdevice] Add a generic concept of a "blocked link"

[netdevice] Add a generic concept of a "blocked link"

When Spanning Tree Protocol (STP) is used, there may be a substantial
delay (tens of seconds) from the time that the link goes up to the
time that the port starts forwarding packets.

Add a generic concept of a "blocked link" (i.e. a link which is up but
which is not expected to communicate successfully), and allow "ifstat"
to indicate when a link is blocked.

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