native: Contiki-NG as a native process

This platform enables running Contiki-NG as a native process, that is, a Linux, OSX or Windows executable. The serial port is then connected to stdin and stdout. Add the shell tutorial:shell to be able to interact with the process.

If the process is started with sufficient permissions, a tun interface will connect the Contiki-NG stack to the host OS. The IPv6 ping example demonstrates this feature in tutorial:ping.