GRUB is used as a loader of the Linux system.  However, Red Hat Enterprise Linux 7 does not ship GRUB anymore.  Instead, it ships GRUB2 only.  Because the configuration system has been completely overhauled, it is not possible to migrate the GRUB setup to GRUB2 automatically.

During the upgrade process, GRUB will be preserved in the Master Boot Record and a temporary GRUB configuration will be deployed to allow you to boot into the upgraded system.

Because it is not possible to support the temporary configuration, you need to rewrite it and install GRUB2 to the Master Boot Record manually after the upgrade.

For details on GRUB2, see the System Administrator's Guide:

    https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/ch-Working_with_the_GRUB_2_Boot_Loader.html
