arch/x86: Add debug spinloop

Conditionally add a debug spinloop to enable easy connection of JTAG
debuggers.

TEST=Build and run on Galileo Gen2 with a JTAG debugger.

Change-Id: I7a21f9e6bfb10912d06ce48447c61202553630d0
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/15127
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/Kconfig

@@ -117,6 +117,13 @@ config PC80_SYSTEM
 	bool
 	default y if ARCH_X86
 
+config BOOTBLOCK_DEBUG_SPINLOOP
+	bool
+	default n
+	help
+	  Add a spin (JMP .) in bootblock_crt0.S during early bootblock to wait
+	  for a JTAG debugger to break into the execution sequence.
+
 config BOOTBLOCK_MAINBOARD_INIT
 	string
 

src/arch/x86/bootblock_crt0.S

@@ -32,6 +32,15 @@
 #include <cpu/x86/16bit/reset16.inc>
 #include <cpu/x86/32bit/entry32.inc>
 
+#if IS_ENABLED(CONFIG_BOOTBLOCK_DEBUG_SPINLOOP)
+
+	/* Wait for a JTAG debugger to break in and set EBX non-zero */
+	xor	%ebx, %ebx
+
+debug_spinloop:
+	cmp	$0, %ebx
+	jz	debug_spinloop
+#endif
 
 bootblock_protected_mode_entry:
 	/* Save BIST result */