[netdevice] Separate VLAN support from presence of VLAN-supporting drivers

Some NICs (e.g. Hermon) provide hardware support for stripping the
VLAN tag, but do not provide any way for this support to be disabled.
Drivers for this hardware must therefore call vlan_find() to identify
a suitable receiving network device.

Provide a weak version of vlan_find() which will always return NULL if
VLAN support has not been enabled (either directly, or by enabling
a feature such as FCoE which requires VLAN support).  This allows the
VLAN code to be omitted from builds where the user has not requested
support for VLANs.

Inspired-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdevice.c

 	return 0;
 }
 
+/**
+ * Identify VLAN device (when VLAN support is not present)
+ *
+ * @v trunk		Trunk network device
+ * @v tag		VLAN tag
+ * @ret netdev		VLAN device, if any
+ */
+__weak struct net_device * vlan_find ( struct net_device *trunk __unused,
+				       unsigned int tag __unused ) {
+	return NULL;
+}
+
 /** Networking stack process */
 PERMANENT_PROCESS ( net_process, net_step );