add: Kconfig assert or errorcode option

add: private include header
add: macro encapsulation for assertion or error check
add: ESP_FAIL return code documentation in public headers
change: replaced all assertions by NVS_ASSERT_OR_RETURN macro
change: few internal function return values from void to esp_err_t
change: ESP_ERR_NVS_VALUE_TOO_LONG macro comment

components/nvs_flash/private_include/nvs_internal.h

@@ -0,0 +1,18 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef NVS_INTERNAL_H
+#define NVS_INTERNAL_H
+
+#if CONFIG_NVS_ASSERT_ERROR_CHECK
+#define NVS_ASSERT_OR_RETURN(condition, retcode) assert(condition);
+#else
+#define NVS_ASSERT_OR_RETURN(condition, retcode) \
+    if (!(condition)) {                          \
+        return retcode;                          \
+    }
+#endif // CONFIG_NVS_ASSERT_ERROR_CHECK
+
+#endif // NVS_INTERNAL_H