Re-enable use of ln -r to drop delta from Debian

Previous versions or Aperits used an old coreutils-gplv2 which did not
support ln -r. After switching to rust-coreutils, ln -r is available,
which allows to drop this delta.

This also fixes issues with systems with not merged usr, like in LAVA
NFS image.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

hook-functions

@@ -200,7 +200,7 @@ copy_file() {
 
 			# Create a relative link so it always points
 			# to the right place
-			ln -s "${link_target}" "${DESTDIR}/${target}"
+			ln -rs "${DESTDIR}/${link_target}" "${DESTDIR}/${target}"
 		fi
 
 		# Copy the link target if it doesn't already exist