[netdevice] Add vlan_tag() to get the VLAN tag of a network device

The iBFT has a VLAN field that should be filled in.  Add the
vlan_tag() function to extract the VLAN tag of a network device.

Since VLAN support is optional, define a weak function that returns 0
when iPXE is built without VLAN support.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[iscsi] Include 802.1Q VLAN identifier in iBFT

The iBFT NIC section has a VLAN field which must be filled in so that
iSCSI booting works over VLANs.

Unfortunately it is unclear from the IBM specification linked in
ibft.c whether the VLAN field is just the 802.1Q VLAN Identifier or
the full 802.1Q TCI.  For now just fill in the VID, the Priority Code
Point and Drop Eligible Indicator could be set in the future if it
turns out they should be present too.

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