net: Use simple UART driver for handling slip (serial line IP) traffic

This commit uses the simple UART driver to pass slip data between
qemu and host.

User needs to create a network device (tun) in host that is
connected to qemu via slip protocol. The tunslip6 process
acts as a gateway and passes packets between qemu and host.
See net/ip/contiki/tools/README file for details how to use and
configure the slip in host.

This is meant for testing the uIP stack.

Change-Id: I82bb87a76485a07ddf5de9ae91fe3d49a0e20658
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>