dsa: config: sample: Add SHELL support via RTT to DSA sample's prj.conf

Due to the routing, the ip_k66f board can only use RTT to export
console. With this change the SHELL is enabled to get access
to network commands (like ping).

To use it with RTT from Segger:
-------------------------------

On HOST (terminal 1):
./JLink_V664/JLinkRTTLogger -Device MK66FN2M0XXX18 -RTTChannel 1 \
-if SWD -Speed 4000 ~/rtt.log

On HOST (terminal 2):
nc localhost 19021

(19021 is the port number for the Segger RTT server)

Signed-off-by: Lukasz Majewski <lukma@denx.de>