cmd: pxe_utils: sysboot: Add zboot support to boot x86 Linux kernel image

Add "zboot" command to the list of supported boot in the
label_boot function.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: add component tags in the summary]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>