[stp] Add support for detecting Spanning Tree Protocol non-forwarding ports

A fairly common end-user problem is that the default configuration of
a switch may leave the port in a non-forwarding state for a
substantial length of time (tens of seconds) after link up.  This can
cause iPXE to time out and give up attempting to boot.

We cannot force the switch to start forwarding packets sooner, since
any attempt to send a Spanning Tree Protocol bridge PDU may cause the
switch to disable our port (if the switch happens to have the Bridge
PDU Guard feature enabled for the port).

For non-ancient versions of the Spanning Tree Protocol, we can detect
whether or not the port is currently forwarding and use this to inform
the network device core that the link is currently blocked.

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

src/config/config_ethernet.c

@@ -40,3 +40,6 @@ REQUIRE_OBJECT ( aoe );
 #ifdef NET_PROTO_FCOE
 REQUIRE_OBJECT ( fcoe );
 #endif
+#ifdef NET_PROTO_STP
+REQUIRE_OBJECT ( stp );
+#endif

src/config/general.h

@@ -37,6 +37,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define	NET_PROTO_IPV4		/* IPv4 protocol */
 #undef	NET_PROTO_IPV6		/* IPv6 protocol */
 #undef	NET_PROTO_FCOE		/* Fibre Channel over Ethernet protocol */
+#define	NET_PROTO_STP		/* Spanning Tree protocol */
 
 /*
  * PXE support