[FIX] Fix warning in Linux driver

Change-Id: Idebd6c02c2c34a7975d27bb3ef52fb09516e8e2a
[FEATURE] Add API event for default gateway change

This feature introduces an API event which is triggered when the default
gateway object (0x1E40/5) is changed.

Change-Id: Idd074984af3de888479f452a17d06560587cf2f5
[TASK] Change ARP demo send request function interface

Change arp_sendRequest function parameter to target IP address. This
enables to send an ARP request to any IP address.

Change-Id: I587022dfebc86bb01c2df575c9c12ba5ac551093
[FEATURE] Add default gateway handling in embedded demos

- Embedded demos handle API event for changed default gateway
- Add default gateway setter in ARP demo
- ARP reply from default gateway is recognized

Change-Id: I6c143d24ed09714ea391e109588fffaa296aadd2