Handle absent udev rules dir

It seems the bug that caused udev rules to be installed in the wrong
location has been fixed.  As such, we need to make this corrective
action step conditional, only moving rules files if any are found in the
wrong place.

build-rootfs.sh

@@ -65,9 +65,11 @@ fi
 
 cp -uv /usr/${target}/usr/bin/grub-editenv /mnt/gentoo/usr/bin/
 
+if [ -d /mnt/gentoo/usr/${target}/usr/lib/udev/rules.d ]; then
     find /mnt/gentoo/usr/${target}/usr/lib/udev/rules.d \
         -name '*.rules' \
         -exec mv -t /mnt/gentoo/usr/lib/udev/rules.d/ {} +
+fi
 
 if [ -f /mnt/gentoo/etc/udev/hwdb.bin ]; then
     mv /mnt/gentoo/etc/udev/hwdb.bin /mnt/gentoo/usr/lib/udev/