x86: coreboot: Remove dead code

start.S does nothing and can be safely removed. Makefile is still being used
by the build system, so simply drop the rule from it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

board/coreboot/coreboot/Makefile

@@ -10,4 +10,4 @@
 # (C) Copyright 2002
 # Daniel Engström, Omicron Ceti AB, daniel@omicron.se.
 
-obj-y	+= start.o coreboot.o
+obj-y	+= coreboot.o

board/coreboot/coreboot/start.S

@@ -1,12 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0+ */
-/*
- * Copyright (c) 2011 The Chromium OS Authors.
- * (C) Copyright 2008
- * Graeme Russ, graeme.russ@gmail.com.
- */
-
-/* board early intialization */
-.globl early_board_init
-early_board_init:
-	/* No 32-bit board specific initialisation */
-	jmp	early_board_init_ret