On the default path, if kernel.org git mirror fails, default to github.com mirror

linux-tkg-config/prepare

@@ -23,6 +23,10 @@ fi
 
 if [[ -z "$_git_mirror" ]]; then
   _git_mirror="${_git_remote_names[2]}"
+  if $( ! timeout 5 git ls-remote ${_kernel_git_remotes[$_git_mirror]} ) || [[ "$( git ls-remote ${_kernel_git_remotes[$_git_mirror]} )" = *502* ]]; then
+    warning "kernel.org unreachable or too long to respond"
+    _git_mirror="${_git_remote_names[0]}"
+  fi
   echo "Defaulting to ${_git_mirror} git mirror"
 fi