NVRAM refactor, phase 1. (#18969)

* Rename `eeprom_stm32` to `eeprom_legacy_emulated_flash`.

* Rename `flash_stm32` to `legacy_flash_ops`.

* Rename `eeprom_teensy` to `eeprom_kinetis_flexram`.

platforms/test/eeprom_legacy_emulated_flash_tests.h

@@ -0,0 +1,8 @@
+// Copyright 2018-2022 Nick Brassel (@tzarc)
+// SPDX-License-Identifier: GPL-2.0-or-later
+#pragma once
+
+#include "legacy_flash_ops.h"
+#include "eeprom_legacy_emulated_flash.h"
+
+#define EEPROM_SIZE (FEE_PAGE_SIZE * FEE_PAGE_COUNT / 2)