Fix setting overlay directory timestamps

Using `tar` to copy files and directories from the overlay directory to
the destination root preserves their timestamps.  This is not really
desirable, particularly for directories, because it may result in the
destination paths appearing older than the build.  This is especially
problematic for `/usr`, since its timestamps are important for systemd
units that use `ConditionNeedsUpdate` settings.

To ensure the timestamps are set correctly, we now use `rsync` to copy
the overlay, with the `-O` (`--omit-dir-times`) argument, to avoid
changing the timestamps of directories.  Additionally, we explicitly
update the timestamp of `/usr` to ensure that every new build triggers
the "needs update" condition.

build-rootfs.sh

@@ -53,8 +53,7 @@ while read name; do
     fi
 done < busybox.symlinks
 
-tar -c -C overlay --owner=root --group=root . \
+rsync -rltpDO overlay/ /mnt/gentoo/
-    | tar -x -C /mnt/gentoo
 
 cp -uv /usr/${target}/usr/bin/grub-editenv /mnt/gentoo/usr/bin/
 
@@ -104,3 +103,5 @@ setfiles \
     -e /mnt/gentoo/etc/portage \
     /mnt/gentoo/etc/selinux/strict/contexts/files/file_contexts \
     /mnt/gentoo
+
+touch /mnt/gentoo/usr