firmware: add mirror courtsey of https://auf-feindgebiet.de/

src/etc/inc/system.inc

@@ -249,6 +249,7 @@ function get_firmware_mirrors()
 
     $mirrors['default'] = '(default)';
     $mirrors['https://opnsense.aivian.org'] = 'Aivian (Shaoxing, CN)';
+    $mirrors['https://mirror.auf-feindgebiet.de/opnsense'] = 'auf-feindgebiet.de (Karlsruhe, DE)';
     $mirrors['https://opnsense.c0urier.net'] = 'c0urier.net (Lund, SE)';
     $mirrors['https://fleximus.org/mirror/opnsense'] = 'Fleximus (Roubaix, FR)';
     $mirrors['http://mirror.ams1.nl.leaseweb.net/opnsense'] = 'LeaseWeb (Amsterdam, NL)';