samples: mesh_demo: Use second button to change target address

It's been observed that that the relay toggling functionality is not
very useful, and that it's better left enabled always. Change the
purpose of the second button to instead modify the target address that
messages sent through the first button get directed to. By default the
destination is the group address, i.e. all nodes receive the message.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>