net: Assign local UDP port number

Assign local UDP port number if not explicitely set by the caller. Use
the timer as a source of uniqueness as there is no random function
available.

If the local UDP port number has been set, verify that it is available
and not already assigned.

Change-Id: I18baac0cbec300cc2a5b7a62dff830d9fafcf2c9
Signed-off-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>