gluon-core: upgrade: don't print an error message on systems without opkg

package/gluon-core/luasrc/lib/gluon/upgrade/500-opkg

@@ -24,6 +24,7 @@ end
 
 local prefix = subst['%%n'] .. '_'
 
+if fs.access('/etc/opkg/distfeeds.conf') then
 	local distfeeds = {}
 	for line in io.lines('/etc/opkg/distfeeds.conf') do
 		table.insert(distfeeds, line)
@@ -56,3 +57,4 @@ if site.opkg and site.opkg.extra and next(site.opkg.extra) then
 	else
 		os.remove('/etc/opkg/gluon.conf')
 	end
+end