netifd: on dhcp interfaces, store the dhcp server in interface data

Among other things, this can be used to auto-configure the DHCP server
address for wireless APs using FILS, if the bridged interface is
configured to DHCP

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/network/config/netifd/files/lib/netifd/dhcp.script

@@ -60,6 +60,7 @@ setup_interface () {
 	[ -n "$message" ]  && json_add_string message "$message"
 	[ -n "$timezone" ] && json_add_int timezone "$timezone"
 	[ -n "$lease" ]    && json_add_int leasetime "$lease"
+	[ -n "$serverid" ] && json_add_string dhcpserver "$serverid"
 	proto_close_data
 
 	proto_send_update "$INTERFACE"