gluon-core: simplify 820-dns-config code

package/gluon-core/luasrc/lib/gluon/upgrade/820-dns-config

 #!/usr/bin/lua
+
 local site = require 'gluon.site_config'
 local uci = require('simple-uci').cursor()
 
-dnsmasq=uci:get_first("dhcp", "dnsmasq")
+local dns = site.dns or {}
+local next_node = site.next_node or {}
+
+local dnsmasq = uci:get_first("dhcp", "dnsmasq")
 
 uci:set('dhcp', dnsmasq, 'localise_queries', true)
 uci:set('dhcp', dnsmasq, 'localservice', false)
 
-if site.dns and site.dns.servers then
-	uci:set('dhcp', dnsmasq, 'server', site.dns.servers)
-else
-	uci:delete('dhcp', dnsmasq, 'server')
-end
+uci:set('dhcp', dnsmasq, 'server', dns.servers)
+uci:set('dhcp', dnsmasq, 'cachesize', dns.cacheentries)
 
-if site.dns and site.dns.cacheentries then
-	uci:set('dhcp', dnsmasq, 'cachesize', site.dns.cacheentries)
-else
-	uci:delete('dhcp', dnsmasq, 'cachesize')
-end
-
-if site.next_node and site.next_node.name and site.next_node.ip4 then
-	uci:section('dhcp','domain','nextnode4',{
-		name=site.next_node.name,
-		ip=site.next_node.ip4,
+if next_node.name and next_node.ip4 then
+	uci:section('dhcp', 'domain', 'nextnode4', {
+		name = next_node.name,
+		ip = next_node.ip4,
 	})
 else
 	uci:delete('dhcp', 'domain', 'nextnode4')
 end
 
-if site.next_node and site.next_node.name and site.next_node.ip6 then
-	uci:section('dhcp','domain','nextnode6',{
-		name=site.next_node.name,
-		ip=site.next_node.ip6,
+if next_node.name and next_node.ip6 then
+	uci:section('dhcp', 'domain', 'nextnode6', {
+		name = next_node.name,
+		ip = next_node.ip6,
 	})
 else
 	uci:delete('dhcp', 'domain', 'nextnode6')
 end
-uci:save('dhcp')
 
+uci:save('dhcp')