net: lwm2m: cleanup token / token length handling

Instead of using a magic reference to 8 for token length, let's
establish a define for MAX_TOKEN_LENGTH and then use it for both
variable definitions and to make sure tokens are valid.  Also,

Correct the handling of a special token length value (0xFF) which
lets lwm2m_init_message() know to skip token generation.  We were
using a -1 value here previously (on a u8_t variable).

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