The macro did not respect zero-length option values hence a pointer
to the option byte itself was returned. This macro now returns NULL
for zero-length option values. Macros and functions that use this
macro have been updated accordingly. COAP_OPT_BLOCK_NUM has been
replaced by the function coap_opt_block_num().
The function wellknown_response() now honors a Block2 option when present
in the request or adds a new Block2 option to the response when the resource
description does not fit.
Bug: get_wkc_len() currently returns a hard-coded value instead of calculating
the size of the resource description (which requires changes in resource.c).
Olaf Bergmann