net: tools: Allow user to set the source address for dtls-client

User can set the source IP address of the sent network packet
in dtls-client by using -b command line option. This helps if
the -i option picks up wrong address from network interface.

Change-Id: I8c26f1a14de2394085dea1d6008eb758a5ce7ad1
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>