samples: net: Use board specific conf file for echo apps

Use by default a board specific config files for both
echo-server and echo-client sample applications.
This makes more sense than using the network technology specific
config file that was used earlier.

Change-Id: I155fc138f4b0ed1166f4e25267513afc3fb6fd76
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>