Current coap library fails to parse or prepare if packet is more
than one fragment. Added support to handle multi fragment packet.
Also well-known/core api used to prepare coap packet and send it
through net context api immediately. This is goind to be problematic
if user doesn't enable net context. Also user can not encrypt coap
packets. Now api will return prepared coap packet to application.
Application will send it to peer.
Jira: ZEP-2210
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>