samples: lwm2m: combine prj_*.conf files

After the LwM2M sample was introduced several HW related defaults were
added to Kconfig.  We no longer need separate prj*.conf files for K64F
and qemu_x86.  Let's combine them into a single prj.conf file and
adjust documentation.

NOTE: This also adjusts README.rst

Signed-off-by: Michael Scott <michael.scott@linaro.org>