samples: lwm2m: Fix default ID length check for DTLS

If the default which is set to CONFIG_BOARD is used, then the lwm2m
client fails to set the client identity as the default maximum length
allowed is only 16 and for some boards this exceeds.

So, increase the default length to accomodate almost all boards. Also
add a build assert for the length validation.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>