config: abstract fs utils to fix esp32 builds

src/FsUtils.h

@@ -0,0 +1,33 @@
+#pragma once
+#include <iterator>
+
+#include <LittleFS.h>
+
+struct filename_iterator: public std::iterator<std::input_iterator_tag, const char*> {
+  public:
+    filename_iterator();
+    filename_iterator(const char* path, const char* suffix);
+    void next();
+    filename_iterator& operator++();
+    filename_iterator& operator++(int) {filename_iterator ret = *this; ++(*this); return ret;}
+    bool operator==(const filename_iterator &other) const { return valid == other.valid;}
+    bool operator!=(const filename_iterator &other) const { return !(*this == other); }
+    const char* operator*() const {
+      if (!valid) {
+        return NULL;
+      }
+      return ret.c_str();
+    }
+
+  private:
+    String ret;
+    bool valid;
+    const char* suffix;
+
+#ifdef BOARD_ESP8266
+    Dir dir;
+#endif
+#ifdef BOARD_ESP32
+    File dir;
+#endif
+};