Fix group owner of overlay files

Need to set the GID owner of all files copied in from the overlay
directory to *root*/`0`, to match the user/UID.

mkrootfs.sh

@@ -48,7 +48,7 @@ cp -p \
     "${srcdir}"/config.txt \
     "${destdir}"/boot/efi
 
-tar --owner root:0 -C "${srcdir}/overlay" -c . \
+tar --owner root:0 --group root:0 -C "${srcdir}/overlay" -c . \
     | tar -C "${destdir}" -x
 chown -R 1000:1000 "${destdir}"/home/user
 if ! grep -q ^user: "${destdir}"/etc/group; then