soc/intel/cannonlake: reduce bootblock size

Reduce the bootblock size to 16KiB from the default 64KiB.
Not all that space is necessary.

Change-Id: I5c15d0af0f85282b84c8983f0a015aeb45c00a07
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/21903
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lijian Zhao <lijian.zhao@intel.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/cannonlake/Kconfig

@@ -146,4 +146,8 @@ config VBOOT
 	select VBOOT_VBNV_CMOS
 	select VBOOT_VBNV_CMOS_BACKUP_TO_FLASH
 
+config C_ENV_BOOTBLOCK_SIZE
+	hex
+	default 0x4000
+
 endif