If you have the following error upon startup, do this to make the system bootable again.
Failed to set MokListRT: Invalid Parameter Something has gone seriously wrong: import_mok_state() failed:Invalid Parameter
Either burn a new CentoS disc, make a boot flash drive, or insert the install ISO to your VM. Boot from the disc and once the menu shows up select the following: Advanced -> Rescue -> Troubleshooting -> Rescue media -> and press 1 when prompted.
In the terminal you should now have, run the following commands:
cp grubx64.efi shimx64.efi
After the reboot be sure to exclude shim* and mokutil* from yum updates (so that it can stay at version shim-x64-12-2.el7.x86_64)
echo 'exclude=shim,mokutil' >> /etc/yum.conf yum update
You should be good to go now. If that does not work try copying CENTOS\GRUBX64.EFI over top of EFI\BOOTX64.EFI in your boot partition.