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/chibios/drivers/eeprom/eeprom_kinetis_flexram.h

@@ -0,0 +1,25 @@
+// Copyright 2022 Nick Brassel (@tzarc)
+// SPDX-License-Identifier: GPL-2.0-or-later
+#pragma once
+
+#include <ch.h>
+#include <hal.h>
+
+#if defined(K20x)
+/* Teensy 3.0, 3.1, 3.2; mchck; infinity keyboard */
+// The EEPROM is really RAM with a hardware-based backup system to
+// flash memory.  Selecting a smaller size EEPROM allows more wear
+// leveling, for higher write endurance.  If you edit this file,
+// set this to the smallest size your application can use.  Also,
+// due to Freescale's implementation, writing 16 or 32 bit words
+// (aligned to 2 or 4 byte boundaries) has twice the endurance
+// compared to writing 8 bit bytes.
+//
+#    ifndef EEPROM_SIZE
+#        define EEPROM_SIZE 32
+#    endif
+#elif defined(KL2x) /* Teensy LC (emulated) */
+#    define EEPROM_SIZE 128
+#else
+#    error Unsupported Teensy EEPROM.
+#endif