boot: Create a function for final pre-boot steps

There are various functions which announce that booting is imminent and
do related preparation. Most of these are arch-specific.

In practice, most archs do the a similar thing. It would be better to
have a common function, with perhaps some events for things that are
really arch- and board-specific.

Create a new function for this. For now, nothing uses it.

Signed-off-by: Simon Glass <sjg@chromium.org>

boot/bootm_final.c

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Cleanup before handing off to the OS
+ *
+ * Copyright 2025 Simon Glass <sjg@chromium.org>
+ */
+
+#include <bootm.h>
+
+void bootm_final(enum bootm_final_t flags)
+{
+}