net: tools: Refactor resolving of the source IP address

The function that resolves the source IP address we need to
use when sending a test network IP packet in Linux host is
refactored a bit. No functionality changes here.

Change-Id: I0f9bc79a9a7f01b382116b969739d7ad3f671751
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>