net: context: Assign a random port number when context is created

Current behaviour has an issue when UDP context is created with local
port number 0, net_conn_input() happens to treat zero port as
a wildcard ("receive packets for all ports"). net_context_bind()
for a UDP context doesn't affect its existing connection in any way.

Proposed solution is, context should be created with a random free
port assigned and bind() updates connection information from context.

Jira: ZEP-1644

Change-Id: Idb3592b58c831d986763312077b0dcdd95850bc9
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>