applications/luci-splash: Get parent interface from ifname option. This is for the new syntax where interfaces are used for aliases. The alias notation (config alias in /etc/config/network) is no longer supported by luci-splash.

git-svn-id: http://svn.luci.subsignal.org/luci/trunk@9661 ab181a69-ba2e-0410-a84d-ff88ab4c47bc