samples/subsys/mgmt/mcumgr/smp_svr: use LitleFS as FS back-end

This patch addapt the sample to using LittleFS as the FS back-end.
After NFFS will be removed this ensures mcumgr FS command functionality.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

smp_svr cleanu