Add support to both IPv4 and IPv6

• Zephyr: Using sockaddr_storage instead of sockaddr_in6 to add support to ipv4 addresses
• Zephyr: add support to ipv4 by checking if CONFIG_NET_IPV4 is defined, and use AF_INET/sockaddr_in or AF_INET6/sockaddr_in6 accordingly.
• Linux: use 0.0.0.0 address and IPv4 coap multicast address if using IPv4
• Add doxygen comment blocks

Relate-to: #15 (closed)

Signed-off-by: Francesco Pham francesco.pham@huawei.com