lib: Add Kconfig option to enable/disable auto fallback control

Under certain conditions, e.g. automated testing, it is useful
to have the payload (e.g. Linux) reset the reboot_bits CMOS
value.  This allows automated recovery in the case of coreboot
starting properly but the payload failing to start due to bad
configuration data provided by the coreboot image under test.

Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Change-Id: Ifc8f565f8292941d90b2e520cc9c5993b41e9cdd
Reviewed-on: http://review.coreboot.org/8698
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)

src/Kconfig

@@ -236,6 +236,14 @@ config BOOTBLOCK_SOURCE
 	default "bootblock_simple.c" if BOOTBLOCK_SIMPLE
 	default "bootblock_normal.c" if BOOTBLOCK_NORMAL
 
+config SKIP_MAX_REBOOT_CNT_CLEAR
+	bool "Do not clear reboot count after successful boot"
+	default n
+	depends on EXPERT
+	help
+	  Do not clear the reboot count immediately after successful boot.
+	  Set to allow the payload to control normal/fallback image recovery.
+
 config UPDATE_IMAGE
 	bool "Update existing coreboot.rom image"
 	default n

src/lib/fallback_boot.c

@@ -22,8 +22,9 @@ static void set_boot_successful(void)
 	byte |= (byte & (1 << 1)) >> 1;
 
 	/* If we are in normal mode set the boot count to 0 */
-	if(byte & 1)
-		byte &= 0x0f;
+	if (!IS_ENABLED(CONFIG_SKIP_MAX_REBOOT_CNT_CLEAR))
+		if(byte & 1)
+			byte &= 0x0f;
 	outb(byte, RTC_PORT(1));
 }
 #else