gluon-autoupdater: make site.conf branch setting optional

Default to the alphabetically smallest branch if none is set in
site.conf.

Also prevent enabling the autoupdater when no branches exist.

package/gluon-autoupdater/check_site.lua

@@ -1,4 +1,4 @@
-need_string(in_site({'autoupdater', 'branch'}))
+need_string(in_site({'autoupdater', 'branch'}), false)
 
 need_table({'autoupdater', 'branches'}, function(branch)
 	need_alphanumeric_key(branch)

package/gluon-autoupdater/luasrc/lib/gluon/upgrade/500-autoupdater

@@ -5,6 +5,8 @@ local uci = require('simple-uci').cursor()
 local unistd = require 'posix.unistd'
 
 
+local min_branch
+
 for name, config in pairs(site.autoupdater.branches()) do
 	uci:delete('autoupdater', name)
 	uci:section('autoupdater', 'branch', name, {
@@ -13,18 +15,26 @@ for name, config in pairs(site.autoupdater.branches()) do
 		good_signatures = config.good_signatures,
 		pubkey = config.pubkeys,
 	})
+
+	if not min_branch or (name < min_branch) then
+		min_branch = name
+	end
 end
 
 if not uci:get('autoupdater', 'settings') then
 	local enabled = unistd.access('/lib/gluon/autoupdater/default_enabled') ~= nil
 
-	local branch = site.autoupdater.branch()
+	local branch = site.autoupdater.branch(min_branch)
 	local f = io.open('/lib/gluon/autoupdater/default_branch')
 	if f then
 		branch = f:read('*line')
 		f:close()
 	end
 
+	if not branch then
+		enabled = false
+	end
+
 	uci:section('autoupdater', 'autoupdater', 'settings', {
 		enabled = enabled,
 		branch = branch,