net: l2: openthread: Parse Border Router messages in correct context.

This commit adds a work item designated for Border Router message
parsing. A generic message structure has been proposed.

If CONFIG_NET_IPV4 is enabled, border router services will start only
after an IPV4 address has been assigned on the backbone interface.
If border router is initialized before DHCPV4 client has been assigned
an address, initial mDNS IPV4 packets will be dropped, because a source
address cannot be retrieved from that specific backbone interface.

Signed-off-by: Cristian Bulacu <[email protected]>

Author: Cristib05