[efi] Add efi_path_vlan() utility function

EFI provides no API for determining the VLAN tag (if any) for a
specified device handle.  There is the EFI_VLAN_CONFIG_PROTOCOL, but
that exists only on the trunk device handle (not on the VLAN device
handle), and provides no way to match VLAN tags against the trunk
device's child device handles.

The EDK2 codebase seems to rely solely on the device path to determine
the VLAN tag for a specified device handle: both NetLibGetVlanId() and
BmGetNetworkDescription() will parse the device path to search for a
VLAN_DEVICE_PATH component.

Add efi_path_vlan() which uses the same device path parsing logic to
determine the VLAN tag.

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

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