cmd: pxe_utils: Replace ifdef by IS_ENABLED

Replace all the macro ifdef by IS_ENABLED.

All of these configs are set in the defconfig files and not in the
include board headers files.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: keep the preprocessor case unchanged]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>