arch/x86: always use _start as entry symbol for all stages

Instead of keeping track of all the combinations of entry points
depending on the stage and other options just use _start. That way,
there's no need to update the arch/header.ld for complicated cases
as _start is always the entry point for a stage.

Change-Id: I7795a5ee1caba92ab533bdb8c3ad80294901a48b
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13882
Tested-by: build bot (Jenkins)
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>

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

@@ -2,6 +2,7 @@
 
 #include <arch/rom_segs.h>
 #include <cpu/x86/post_code.h>
+#include <rules.h>
 
 	.code32
 
@@ -44,10 +45,17 @@ gdt_end:
  *
  *	NOTE aligned to 4 so that we are sure that the prefetch
  *	cache will be reloaded.
+ *
+ *	In the bootblock there is already a ljmp to __protected_start and
+ *	the reset vector jumps to symbol _start16bit in entry16.inc from
+ *	the reset vectors's symbol which is _start. Therefore, don't
+ *	expose the _start symbol for bootblock.
  */
 	.align	4
-.globl protected_start
-protected_start:
+#if !ENV_BOOTBLOCK
+.globl _start
+_start:
+#endif
 
 	lgdt	%cs:gdtptr
 	ljmp	$ROM_CODE_SEG, $__protected_start