samples: lwm2m: replace prj_dtls.conf with overlay fragment

Instead of maintaining a nearly identical copy of prj.conf to support
DTLS, let's replace it with an overlay-dtls.conf file containing only
the CONFIG settings needed to support DTLS.  This can be used via the
-DCONF_FILE command line argument when configuring the sample:
Non-DTLS-enabled build:
cmake -GNinja -DBOARD=qemu_x86 ..
DTLS-enabled build:
cmake -GNinja -DBOARD=qemu_x86 \
  -DCONF_FILE="prj.conf overlay-dtls.conf" ..

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