gluon-autoupdater: mirror URLs must start with http://

The older busybox-based wget erroneously accepted URLs without protocol.
Add validator to avoid building firmwares with broken autoupdates.

package/gluon-autoupdater/check_site.lua

@@ -6,7 +6,7 @@ local function check_branch(k, _)
    local prefix = string.format('autoupdater.branches[%q].', k)
 
    need_string(prefix .. 'name')
-   need_string_array(prefix .. 'mirrors')
+   need_string_array_match(prefix .. 'mirrors', '^http://')
    need_number(prefix .. 'good_signatures')
    need_string_array_match(prefix .. 'pubkeys', '^%x+$')
 end