[nrf fromtree] samples/fs/littlefs: Use flash_area_flatten to wipe storage

The commit replaces flash_area_erase with flash_area_flatten,
as it allows to emulate erase and scramble data stored
on devices that do not require explicit erase before write
or do not provide erase callback.

Signed-off-by: Dominik Ermel <[email protected]>
(cherry picked from commit e31b8c8)

Author: de-nordic

samples/subsys/fs/littlefs/src/main.c

@@ -252,7 +252,7 @@ static int littlefs_flash_erase(unsigned int id)
 
 	/* Optional wipe flash contents */
 	if (IS_ENABLED(CONFIG_APP_WIPE_STORAGE)) {
-		rc = flash_area_erase(pfa, 0, pfa->fa_size);
+		rc = flash_area_flatten(pfa, 0, pfa->fa_size);
 		LOG_ERR("Erasing flash area ... %d", rc);
 	}