grub-mkimage help needed

[larry]

I have a custom iso based on Ubuntu 16.04 that is designed to run on a 
USB that boots in both Legacy (CSM) and EFI (with CSM support) modes 
just fine. When booting in EFI mode with CSM support, it's using the 
menu as defined in isolinux.cfg instead of the menu defined in grub.cfg, 
which I expected. It won't boot in EFI mode without CSM support at all.

I mounted my custom iso's efi partition and all it contains is 
bootx64.efi as a text file and no grubx64.efi. When I mount the efi 
partition from Ubuntu 16.04's iso, it has both bootx64.efi and 
grubx64.efi as programs. I'm pretty sure that I screwed-up the 
formatting of grub-mkimage and would appreciate some help on this. After 
mounting the iso, my method of modifying it is:

|BOOT_IMG_DATA="$PWD" BOOT_IMG=efi.img mkdir -p $(dirname $BOOT_IMG) 
truncate -s 4M $BOOT_IMG mkfs.vfat $BOOT_IMG mkdir -p 
$BOOT_IMG_DATA/EFI/BOOT grub-mkimage \ -C xz \ -d 
"$HOME"/foxbuild2/src/livecd/image/boot/grub/x86_64-efi \ -O x86_64-efi 
\ -o $BOOT_IMG_DATA/EFI/BOOT/bootx64.efi \ boot linux search normal 
configfile \ part_gpt btrfs fat iso9660 loopback \ test keystatus 
gfxmenu regexp probe \ efi_gop efi_uga all_video gfxterm font \ echo 
read ls cat png jpeg halt reboot mcopy -i $BOOT_IMG -s 
$BOOT_IMG_DATA/EFI :: |

# Create the new ISO image

|# The example names get mapped to their roles here 
#orig_iso=../foxclone.iso new_iso=../foxclone025-4.15UEFI.iso 
new_files="$PWD" mbr_template=/usr/lib/ISOLINUX/isohdpfx.bin xorriso -as 
mkisofs "$new_files" \ -o "$new_iso" \ -isohybrid-mbr 
/usr/lib/ISOLINUX/isohdpfx.bin \ -c isolinux/boot.cat \ -b 
isolinux/isolinux.bin \ -no-emul-boot -boot-load-size 4 -boot-info-table 
\ -eltorito-alt-boot \ -e isolinux/efi.img \ -no-emul-boot \ 
-isohybrid-gpt-basdat \ -m grub.cfg |

Thanks in advance, Larry