I'm at the same point, following the same wiki page. I was able to determine that the file called startup.nsh should go in /boot/efi which you can access by mounting the correct partition after the dd command has been completed.... ie
dd if=CentOS-7-aarch64.img of=/dev/sdc bs=2M conv=fsync && sync;
This command creates 4 partitions on the device, in my example, /dev/sdc is my microSD card
p1 = loader
p2 = /boot
p3 = swap
p4 = /root
you can mount each one with "mount /dev/sdc2 /mnt", as an example ( of course, not the swap )
I mounted p2, and created a file in the efi directory called startup.nsh with the following in it:
Image console=ttyAMA3,115200 root=/dev/sda4 rootwait rw efi=noruntime
The example gives this:
Image console=ttyAMA3,115200 root=/dev/mmcblk1p4 rootwait rw efi=noruntime
and I'm simply making an assumption it will see the MicroSD as a "sd" styled device.
the root partition in a similar fashion would be then mounted like "mount /dev/sdc4 /mnt1"
But there is no indication where to get xen.efi, and if found, where on the mounted root fs it needs to go, as well as where the xen.cfg file is suppose to reside.