move netdev and netlink into their own packages and out of drivers

This moves the netdev/netlink namespace out of drivers and into their
own packages.  Also, defines netdev and netlink as L3/L4 and L2 OSI
layers, respectively.  Move some L3 functionality from netlink to
netdev (GetIPAddr).

For netlink, add ConnectParams for NetConnect to pass in L2 connection
parameters (ssid, pass, auth_type, etc).  Also adds connection mode
(STA, AP, etc).

For netlink, add SendEth and RecvEthFunc funcs to handle L2 send/recv of
Ethernet pkts.

netlink/probe/rtl8720dn.go

@@ -0,0 +1,29 @@
+//go:build wioterminal
+
+package probe
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/netdev"
+	"tinygo.org/x/drivers/netlink"
+	"tinygo.org/x/drivers/rtl8720dn"
+)
+
+func Probe() (netlink.Netlinker, netdev.Netdever) {
+
+	cfg := rtl8720dn.Config{
+		// Device
+		En: machine.RTL8720D_CHIP_PU,
+		// UART
+		Uart:     machine.UART3,
+		Tx:       machine.PB24,
+		Rx:       machine.PC24,
+		Baudrate: 614400,
+	}
+
+	rtl := rtl8720dn.New(&cfg)
+	netdev.UseNetdev(rtl)
+
+	return rtl, rtl
+}