netifd: replace backticks by $(...)

This replaces deprecated backticks by more versatile $(...) syntax.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/network/config/netifd/files/sbin/ifup

@@ -37,7 +37,7 @@ done
 
 [ "$modes" = "down up" ] && ubus call network reload
 if [ -n "$ifup_all" ]; then
-	for interface in `ubus -S list 'network.interface.*'`; do
+	for interface in $(ubus -S list 'network.interface.*'); do
 		if_call "${interface##network.interface.}"
 	done
 	[ -n "$setup_wifi" ] && /sbin/wifi up