cpu/x86: Link entry16.inc

Change-Id: I78ecd15716169b58cf6696ff8c5069ac2d5038ef
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47967
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/32bit/entry32.inc

@@ -13,6 +13,7 @@
  */
 	.align	4
 
+.globl __protected_start
 __protected_start:
 	/* Save the BIST value */
 	movl	%eax, %ebp

src/cpu/x86/Makefile.inc

@@ -8,6 +8,8 @@ ramstage-y += backup_default_smm.c
 
 subdirs-$(CONFIG_CPU_INTEL_COMMON_SMM) += ../intel/smm
 
+bootblock-y += entry16.S
+
 additional-dirs += $(obj)/cpu/x86
 
 SIPI_ELF=$(obj)/cpu/x86/sipi_vector.elf

src/cpu/x86/entry16.S

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+
 /*
  * This software and ancillary information (herein called SOFTWARE)
  * called LinuxBIOS is made available under the terms described here.
@@ -28,8 +30,10 @@
 #include <arch/rom_segs.h>
 #include <cpu/x86/post_code.h>
 
-/* Symbol _start16bit must be aligned to 4kB to start AP CPUs with
- * Startup IPI message without RAM.
+.section .init._start, "ax", @progbits
+
+/* Symbol _start16bit must reachable from the reset vector, and be aligned to
+ * 4kB to start AP CPUs with Startup IPI message without RAM.
  */
 .align 4096
 .code16