samples: net: Use context specific net_buf pools in echo_*

Use context specific net_buf pool in echo-server and echo-client.
This will prevent application from allocating all available network
buffers in the system and thus preventing the core IP stack
functionality.

Change-Id: I3dbc64dd81c8a1165241426287a3530d00cbc7bf
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>