[patches] use luci.model.ipkg instead of luci.fs to check if wizard is installed

A cleaner way to determine if the wizard is installed is to check if the packet
is installed via ipkg. With this we can change the wizard without breaking the
patch.
Merge pull request #23 from cholin/firstboot_patch

[patches] use luci.model.ipkg instead of luci.fs to check if wizard is installed