netlink: add L2 bridge/bond/vlan intermediate drivers

This adds three new L2 (intermediate) drivers for bridge/bond/vlan.
Theses drivers are incomplete but illustrate how to stack L2 drivers.
Here are some examples of how these driver would stack with the cy243439
driver:

  Bridge: bridge two cyw43 devices together, connecting the two LANs:

	cyw43_0 := cyw43439.NewDevice(...)
	cyw43_1 := cyw43439.NewDevice(...)

	bridge := NewBridge([]netlink.Netlinker{cyw43_0, cyw43_1})

	stack := tcpip.NewStack(bridge)
	netdev.UseNetdev(stack)

  Bond: bond two cyw43 devices together, creating one logical device.
  The first physical device is primary, the second is backup (fail-over)
  device:

	cyw43_0 := cyw43439.NewDevice(...)  // primary
	cyw43_1 := cyw43439.NewDevice(...)  // secondary (backup)

	bond := NewBond([]netlink.Netlinker{cyw43_0, cyw43_1})

	stack := tcpip.NewStack(bond)
	netdev.UseNetdev(stack)

   Vlan: add tagged VLAN ID=100 to cyw43:

	cyw43 := cyw43439.NewDevice(...)

	vlan100 := NewVlan(100, cyw43)

	stack := tcpip.NewStack(vlan100)
	netdev.UseNetdev(stack)

netlink/netlink.go

@@ -99,5 +99,5 @@ type Netlinker interface {
 
 	// RecvEth callback function for receiving Ethernet pkt
 	// TODO describe content of pkt
-	RecvEthFunc(func(pkt []byte) error)
+	RecvEthFunc(cb func(pkt []byte) error)
 }