espnet: WIP support for on-chip WiFi on an ESP32C3

Work in progress. Does not work yet.

Some notes:

  - This requires some changes to TinyGo, look at the espnet branch.
  - The  next step is probably defining all the functions in
    g_wifi_osi_funcs (see espnet.c). Right now it hangs in
    esp_wifi_init_internal, probably a NULL pointer dereference.
  - This is only for the ESP32-C3. This will require some work to work
    on other chips from Espressif.

examples/espnet/main.go

@@ -0,0 +1,20 @@
+package main
+
+import "tinygo.org/x/drivers/espnet"
+
+func main() {
+	err := espnet.WiFi.Configure(espnet.Config{})
+	if err != nil {
+		println("failed to configure:", err.Error())
+	}
+	mac, err := espnet.WiFi.AccessPointMAC()
+	if err != nil {
+		println("failed to read MAC address:", err.Error())
+		return
+	}
+	print("MAC address:")
+	for _, b := range mac {
+		print(" ", b)
+	}
+	println()
+}