ethernet.h could include defines for magic numbers, for common Ethernet frame types

The app-note code often includes things like,


```
  cfg.add_ethertype_filter(index, 0x0806);
  cfg.add_ethertype_filter(index, 0x0800);
```

Where ideally there should be the following defines, or similar,

```
#define ETH_FRAME_TYPE_ARP 0x0806
#define ETH_FRAME_TYPE_IP  0x0800
```
