Stage SELinux config file in the sysroot.

This is needed to build policy modules outside of refpolicy. Their
build systems need to determin the name of the policy that will be
in effect on the target host. This allows them to locate the policy
headers that will be under $sysroot/usr/share/selinux/$name/include.
Given that there *could* be more than one policy installed in the
sysroot we can't assume that the policy installed there is the only
policy to build against.
Add recipe to build example policy module.
Add bbappend for 3.10 kernel.
Add sysvinit to DISTRO_FEATURES in oe-selinux distro.

The oe-selinux distro won't build against the master branch of
oe-core unless sysvinit is in DISTRO_FEATURES.
Commit c72ec4b52827f75351790eab483d258b2e87611a in oe-core adds a
sanity check to the packagegroup class that requires the
VIRTUAL-RUNTIME_init_manager explicitly be set in DISTRO_FEATURES.