system: time zones are deleted from tzdata so fallback gracefully

(cherry picked from commit 015c5d00)
(cherry picked from commit a9f41407)

src/etc/inc/system.inc

@@ -1394,16 +1394,15 @@ function system_timezone_configure()
     $timezone = $syscfg['timezone'];
     $timezones = get_zoneinfo();
 
-    // Reset to default if empty or not existend
-    if (empty($timezone) || !in_array($timezone, $timezones)) {
+    /* reset to default if empty or nonexistent */
+    if (empty($timezone) || !in_array($timezone, $timezones) ||
+        !file_exists(sprintf('/usr/share/zoneinfo/%s', $timezone))) {
         $timezone = 'Etc/UTC';
     }
 
-    // Apply timezone
+    /* apply timezone */
     copy(sprintf('/usr/share/zoneinfo/%s', $timezone), '/etc/localtime');
 
-    mwexec("sync");
-
     if (file_exists("/var/run/booting")) {
         echo gettext("done.") . "\n";
     }