Only use SELinux tools from toolbox if not in busybox

If busybox has SELinux support, we don't need toolbox to provide SELinux
specific tools. Check busybox for restorecon to see if SELinux support
is present and then adjust the tools built by toolbox based on this.

Change-Id: I30a2b036b97ac3e26510d5e0498c52d7a8af36e1