Add "kern_guid=%U" to the secure boot kernel command-line template.

With the newest Chrome OS BIOS and bootstub, this will be expanded to the
booted kernel partition's UniqueGuid, so that the kernel device can be
determined with certainty, since the BIOS and kernel may enumerate drives
differently.

You can identify the booted kernel partition at runtime with something like
this:

  sudo cgpt find -1 -u \
    $(cat /proc/cmdline | sed 's/.*kern_guid=\([0-9a-f-]\+\).*/\1/')

Review URL: http://codereview.chromium.org/3035020

build_kernel_image.sh

@@ -128,6 +128,7 @@ noswap
 i915.modeset=1
 loglevel=7
 cros_secure
+kern_guid=%U
 EOF
   WORK="${WORK} ${FLAGS_working_dir}/config.txt"