Add details about generating a grub config for RHEL8/EFI targets

docs/migration.md

@@ -43,7 +43,12 @@ Please follow these commands:
 
 1. `dnf distro-sync --allowerasing --disablerepo=openafs*`
 2. `dnf distro-sync`
-3. `reboot`
+
+!!! danger "If you are migrating from CentOS Stream 8 to RHEL8 (and your system is EFI), you must also rebuild the grub configuration. Failing to do this will result in your system being unable to boot. If your target is RHEL9 or AlmaLinux (8 or 9), you do not need to perform this step"
+
+1. `grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg`
+
+You may now `reboot` the system
 
 !!! danger "If you encounter a 'transaction error' - please refer to the Troubleshooting section below"
 
@@ -86,8 +91,12 @@ Migration with no reinstall for CERN IT Puppet-managed machines was enabled by t
     ```
     $ mco shell run "systemctl start --wait dnf-distro-sync.service" --batch 100 --batch-sleep 60 -t 5 -T lxsoft -F hostgroup_0=lxsoft
     ```
-2. Reboot.
-3. Once you're happy, add your machine(s) back into production.
+
+!!! danger "If you are migrating from CentOS Stream 8 to RHEL8 (and your system is EFI), you must also rebuild the grub configuration. Failing to do this will result in your system being unable to boot. If your target is RHEL9 or AlmaLinux (8 or 9), you do not need to perform this step"
+
+1. `grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg`
+
+You may now `reboot` the system and once you're happy, add your machine(s) back into production.
 
 The first Puppet run is needed so Facter reports this machine as the 'new' operating system
 to the Puppet servers, the second Puppet run is the one that actually configures