Added a freeEntries method to Preferences library (espressif#1722)

Author: lbernstone

libraries/Preferences/src/Preferences.cpp

@@ -468,3 +468,13 @@ size_t Preferences::getBytes(const char* key, void * buf, size_t maxLen){
     }
     return len;
 }
+
+size_t Preferences::freeEntries() {
+    nvs_stats_t nvs_stats;
+    esp_err_t err = nvs_get_stats(NULL, &nvs_stats);
+    if(err){
+        log_e("Failed to get nvs statistics");
+        return 0;
+    }
+    return nvs_stats.free_entries;
+}

libraries/Preferences/src/Preferences.h

@@ -64,6 +64,7 @@ class Preferences {
         size_t getString(const char* key, char* value, size_t maxLen);
         String getString(const char* key, String defaultValue = String());
         size_t getBytes(const char* key, void * buf, size_t maxLen);
+        size_t freeEntries();
 };
 
 #endif