Deprecates NVStore

Author: Veijo Pesonen

features/storage/nvstore/source/nvstore.h

@@ -294,6 +294,7 @@ class NVStore : private mbed::NonCopyable<NVStore> {
     uint8_t *_page_buf;
 
     // Private constructor, as class is a singleton
+    MBED_DEPRECATED_SINCE("mbed-os-5.15", "NVStore is deprecated in favor of KVStore")
     NVStore();
 
     /**