net: lwm2m: setup LwM2M build as static library

Instead of building under the "app" context, let's build the
LwM2M library as a separate static library.  This will be helpful
later when adding support for DTLS as w/o this configuration,
the build breaks on MBEDTLS config includes.

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