use ip syntax according to ip-address(8)

According to ip-address(8) flushing an interface requires the
keyword 'dev'.
Also add proper quoting.

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

hooks/archiso_pxe_common

@@ -51,8 +51,8 @@ run_latehook () {
         [[ -z "${copy_resolvconf}" ]] && copy_resolvconf="y"
 
         if [[ "${copytoram}" == "y" ]]; then
-            ip addr flush ${bootif_dev}
-            ip link set ${bootif_dev} down
+            ip addr flush dev "${bootif_dev}"
+            ip link set "${bootif_dev}" down
         elif [[ "${copy_resolvconf}" != "n" && -f /etc/resolv.conf ]]; then
             cp /etc/resolv.conf /new_root/etc/resolv.conf
         fi