You have ton install efi version of syslinux on your flashdrive.
I think we will add efi support to the boot loader in next release.
in this thread: viewtopic.php?f=2&t=5026&start=2030#p67329
Like Trantor say, I create my boot USBkey with syslinux efi bootloader.
But I have error when load the Zimage kernel
I have the same error with the last XPEnoboot 5.2-5967.1
And here is my KEY and syslinux.conf (syslinux 6.04 pre1)
My computer only boot with syslinux.efi renamed to BOOTX64.efi (and 64bit)