The CoAP samples use an MBEDTLS config "config-coap.h" which could be
re-used by the LwM2M sample, except that most servers use a larger
maximum content length setting of 1500 bytes.
Let's add a CONFIG to set this for users of the CoAP lib and set the
CONFIG value for the samples to the 256 size currently used.
Signed-off-by: Michael Scott <michael@opensourcefoundries.com>