cpu/x86/entry16.S: Move reset vector to this file

This makes the code easier to follow.

Change-Id: I5a4b7fe99875a1addf611f569990ff9a3beda3ba
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74800
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Shuo Liu <shuo.liu@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/Makefile.mk

@@ -34,7 +34,6 @@ subdirs-$(CONFIG_CPU_INTEL_COMMON_SMM) += ../intel/smm
 
 bootblock-y += entry32.S
 bootblock-y += entry16.S
-bootblock-y += reset16.S
 
 additional-dirs += $(obj)/cpu/x86
 

src/cpu/x86/entry16.S

@@ -151,3 +151,9 @@ nullidt:
 	.word	0	/* limit */
 	.long	0
 	.word	0
+
+  /* This is the first instruction the CPU runs when coming out of reset. */
+.section ".reset", "ax", %progbits
+.globl _start
+_start:
+	jmp		_start16bit

src/cpu/x86/reset16.S

@@ -1,7 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0-only */
-
-	.section ".reset", "ax", %progbits
-	.code16
-.globl	_start
-_start:
-	jmp   _start16bit