Network interface improvement

Hi all,

Some improvements on the network interface have been made on OpenCN targets.

DHCP support has been enabled: if the OpenCN target is connected to a netwrok which has a DHCP server, it gets it IP from it.
If no DHCP present on the network, the OpenCN target falllback to a static IP and acts as DHCP server. It is the case, for example, when the target is directly connected to a PC.

Support of ssh and avahi has also been enabled to simply communication with the OpenCN target.

With ssh it is possible to log on the target. It is a good complement of the serial interface. It also offers a simple way to copy files between the host and the target with scp or sshfs tools.

avahi lets uses to connect to the target using target HOSTNAME instead of the IP.

The documentation on how to use these different tools can be find here