Include selinux-permissive element for fedora

There is currently an issue with building fedora 20 images
that are selinux enabled. This change modifies the instructions
for building a software-config image to include the
selinux-permissive element, which allows image building to
complete.

Change-Id: Ib67dc5a30a4e1f25e282efbfefc89dd816f7be4e
Related-Bug: #1316241
Merge "Include selinux-permissive element for fedora"