gluon-mesh-vpn-fastd: merge secret generation setup into main upgrade script

There wasn't really a reason to have a separate script to set a single
value.

In addition, the old script was using the identifier 'c' instead of
'uci' for the UCI cursor. Following the convention of the other scripts
is helpful so it is easy to grep for all uses of a certain config file/
option.

package/gluon-mesh-vpn-fastd/luasrc/lib/gluon/upgrade/400-mesh-vpn-fastd

@@ -9,6 +9,11 @@ local uci = require('simple-uci').cursor()
 
 local syslog_level = uci:get('fastd', 'mesh_vpn', 'syslog_level') or 'verbose'
 
+local secret = uci:get('fastd', 'mesh_vpn', 'secret')
+if not secret or not secret:match(('%x'):rep(64)) then
+	secret = 'generate'
+end
+
 local methods
 
 if site.mesh_vpn.fastd.configurable(false) then
@@ -38,6 +43,7 @@ end
 uci:section('fastd', 'fastd', 'mesh_vpn', {
 	group = 'gluon-mesh-vpn',
 	syslog_level = syslog_level,
+	secret = secret,
 	interface = vpn_core.get_interface(),
 	mode = 'tap',
 	mtu = site.mesh_vpn.mtu(),

package/gluon-mesh-vpn-fastd/luasrc/lib/gluon/upgrade/410-mesh-vpn-fastd-generate-secret

-#!/usr/bin/lua
-
-local uci = require 'simple-uci'
-
-local c = uci.cursor()
-
-local secret = c:get("fastd", "mesh_vpn", "secret")
-
-if not secret or not secret:match(("%x"):rep(64)) then
-  c:set("fastd", "mesh_vpn", "secret", "generate")
-  c:save("fastd")
-end