slip: Add driver for host to qemu connectivity

The SLIP driver is using UART pipe driver to communicate with
host when Zephyr is running in Qemu. Currently this is only
used by networking sub-system when testing the networking
stack.

Change-Id: I432b4136670766a7fc190f146057924c266f9bea
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>