samples: lwm2m: add overlay conf to support BT networking

The file overlay-bt.conf contains only the needed CONFIG items to
support Bluetooth networking.

For example, DTLS-enabled BLENano2 HW can be supported with the
following build:
cmake -GNinja -DBOARD=nrf52_blenano2 \
  -DCONF_FILE="prj.conf overlay-bt.conf overlay-dtls.conf" ..

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>