Fix vboot_reference-firmware compilation error when building for coreboot.

When compiling for coreboot the printf format helpers are not
available (they come from the Insyde tree).

The specifier is use in a very limited number of places, it is
probably better to typecast the variable being printed to avoid
compilation errors. This CL accomplishes just that.

BUG=none
TEST=manual:
run the following commands:

emerge-x86-alex -C sys-boot/chromeos-coreboot \
    sys-boot/chromeos-u-boot\
    chromeos-base/vboot_reference \
    chromeos-base/vboot_reference-firmware
emerge-x86-alex chromeos-bootimage

observe the second one succeed.

Change-Id: If19e3a583eb759ba5a21863d1b9b28636c7f00b0
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/4690
diff --git a/firmware/lib/vboot_kernel.c b/firmware/lib/vboot_kernel.c
index b3c06b3..64c5cc5 100644
--- a/firmware/lib/vboot_kernel.c
+++ b/firmware/lib/vboot_kernel.c
@@ -385,7 +385,7 @@
         lowest_version = combined_version;
       else {
         VBDEBUG(("Key block valid: %d\n", key_block_valid));
-        VBDEBUG(("Combined version: %" PRIu32 "\n", combined_version));
+        VBDEBUG(("Combined version: %u\n", (unsigned) combined_version));
       }
 
       /* If we already have a good kernel, no need to read another