flush and set down only if boot interface is known

Signed-off-by: Christian Hesse <mail@eworm.de>

archiso/initcpio/hooks/archiso_pxe_common

@@ -52,8 +52,10 @@ run_latehook () {
         [[ -z "${copy_resolvconf}" ]] && copy_resolvconf="y"
 
         if [[ "${copytoram}" == "y" ]]; then
-            ip addr flush dev "${bootif_dev}"
-            ip link set "${bootif_dev}" down
+            if [[ -n "${bootif_dev}" ]]; then
+                ip addr flush dev "${bootif_dev}"
+                ip link set "${bootif_dev}" down
+            fi
         elif [[ "${copy_resolvconf}" != "n" && -f /etc/resolv.conf ]]; then
             cp /etc/resolv.conf /new_root/etc/resolv.conf
         fi