samples: net: lwm2m: use real temperature values instead of dummy

This changes the use of a static and fixed temperature value to using a
read_callback which will either return the previous dummy value or
gather real temperature data from a supported sensor (for now only
FXOS8700 which is available e.g. on a FRDM-K64F).

Signed-off-by: Daniel Egger <daniel@eggers-club.de>