samples: net: lwm2m_client: expand sanity testing

Let's expand the sanity build testing for the lwm2m_client by
doing the following:

- Add variant builds for DTLS, bluetooth and modem
- Check various combinations of hardware across each of these
  variant builds: frdm_k64f, nrf52_pca10040, nrf52840_pca10056,
  disco_l475_iot1

Signed-off-by: Michael Scott <mike@foundries.io>