init-storage: Copy file contexts from rootfs

Apparently, BusyBox's `cp` does NOT copy SELinux contexts when the `-a`
argument is specified.  This differs from GNU coreutils's `cp`, and
explains why the files copied from the rootfs image to the persistent
storage volume were not being labelled correctly.  The `-c` argument is
required.

Now that files are labelled correctly when they are copied, the step to
run `restorecon` is no longer necessary.

overlay/usr/lib/systemd/system/restorecon.service

@@ -1,15 +0,0 @@
-[Unit]
-Description=Restore SELinux file contexts in /var
-ConditionNeedsUpdate=/var
-DefaultDependencies=no
-After=local-fs.target
-Before=sysinit.target
-Before=systemd-tmpfiles-setup.service
-Before=systemd-update-done.service
-
-[Service]
-Type=oneshot
-ExecStart=/usr/sbin/restorecon -RFv /var
-
-[Install]
-WantedBy=sysinit.target