Add Make variables that can be used to override default network settings

e.g.:

```
#ifndef IP_ADDR
#define IP_ADDR DEFAULT_IP_ADDR
#endif

#ifndef SUB_MASK
#define SUB_MASK DEFAULT_SUB_MASK
#endif

#ifndef GW_ADDR
#define GW_ADDR DEFAULT_GW_ADDR
#endif

#ifndef MAC_ADDR
#define MAC_ADDR DEFAULT_MAC_ADDR
#endif
```

Document these variables in docs/network_configuration.md