6rd: Select first IPv4 address from configured tunlink interface as local tunnel address

Gives the user the control to select the correct WAN IPv4 address to be used by the 6rd tunnel when mutiple WAN interfaces are configured

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40566 3c298f89-4303-0410-b956-a3cf2f4a3e73