net: ip: Allow user to select number of network contexts

The network context is similar concept as network socket.
Depending on application use, the number of contexts can be
now tweaked. Default value is now 2, it was 5 earlier. Using
the lower number saves 241 bytes of memory.

Change-Id: I6e2a9f053c8c63163af6d6175783233f67553ae9
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>