* new DynamicConfigSerializer

* board/variant KeyValueStore now can write to 'custom' object in Json prefs

src/helpers/DynamicConfigSerializer.h

@@ -0,0 +1,20 @@
+#include "ConfigSerializer.h"
+#include "KeyValueStore.h"
+
+#ifndef MAX_DYNAMIC_CONFG
+  #define MAX_DYNAMIC_CONFG  128
+#endif
+
+class DynamicConfigSerializer : public ConfigSerializer, public KeyValueStore {
+  char _config[MAX_DYNAMIC_CONFG];
+  KeyValueStore* _fallback;
+
+protected:
+  void structure() override;
+
+public:
+  DynamicConfigSerializer(KeyValueStore* fallback = NULL) : _fallback(fallback) { _config[0] = 0; }
+
+  bool setByKey(const char* key, const char* value) override;
+  bool getByKey(const char* key, char* value, size_t max_len) override;
+};