net: Moved net/ to subsys/net

* Moved networking code into subsys/net.
* Renamed net/yaip to net/ip at the same time.
* Fixed the tests/net to compile
* Fixed the Makefiles and Kconfig files in subsys/net
  to use the new location of the IP stack

Change-Id: Ie45d9e8cb45a93fefdf969b20a81e3b1d3c16355
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>