samples: Add PSK support for echo client/server

PSK support is added to echo-client.c and echo-server.c.
If enabled, a header is included which contains the PSK.
If the default dummy PSK header is used, a warning is issued.
The header can be changed via Kconfig.

Signed-off-by: Torstein Grindvik <torstein.grindvik@nordicsemi.no>