feat: Add function to erase all settings

Added a zmk_settings_erase() function to clear all saved settings. This
does not go through Zephyr's settings subsystem, but instead directly
clears the data from the setting storage backend, so a reboot is needed
for it to take effect.

app/src/settings/reset_settings_nvs.c

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/device.h>
+#include <zephyr/devicetree.h>
+#include <zephyr/storage/flash_map.h>
+
+#include <zephyr/logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+// SETTINGS_PARTITION must match settings_nvs.c
+#if DT_HAS_CHOSEN(zephyr_settings_partition)
+#define SETTINGS_PARTITION DT_FIXED_PARTITION_ID(DT_CHOSEN(zephyr_settings_partition))
+#else
+#define SETTINGS_PARTITION FIXED_PARTITION_ID(storage_partition)
+#endif
+
+int zmk_settings_erase(void) {
+    LOG_INF("Erasing settings flash partition");
+
+    const struct flash_area *fa;
+    int rc = flash_area_open(SETTINGS_PARTITION, &fa);
+    if (rc) {
+        LOG_ERR("Failed to open settings flash: %d", rc);
+        return rc;
+    }
+
+    rc = flash_area_erase(fa, 0, fa->fa_size);
+    if (rc) {
+        LOG_ERR("Failed to erase settings flash: %d", rc);
+    }
+
+    flash_area_close(fa);
+
+    return rc;
+}