When booting with GRUB, it is important that the core image and modules
have matching versions. Usually, running grub-install is sufficient to
ensure this.

On the UEFI platform, grub-install allows the core image to be placed in
two different locations:

EFI/gentoo/grubx64.efi
This is the location used by grub-install without options.

EFI/BOOT/BOOTX64.EFI
This is the location used by grub-install --removable.

On upgrades, it is common for users to mismatch the grub-install options
they used for the current and previous versions of grub. This will cause
a stale core image to exist. For example:

/boot/efi/EFI/BOOT/BOOTX64.EFI (grub 2.06 core image)
/boot/efi/EFI/gentoo/grubx64.efi (grub 2.12 core image)
/boot/grub/x86_64-pc/*.mod (grub 2.12 modules)

Booting this system using BOOTX64.EFI image would likely fail due to a
symbol mismatch between the core image and modules.

Re-runing grub-install both with and without the --removable options
should ensure a working GRUB installation.

However, this will clobber any BOOTX64.EFI image provded by other
loaders. If dual-booting using another boot loader, users must take care
not to replace BOOTX64.EFI if it is not provided by GRUB.