- Adicione
GRUB_DISABLE_SUBMENU=ya/etc/default/grub - Regenere a configuração do grub:
grub-mkconfig -o /boot/grub/grub.cfg
Veja o wiki do Arch Linux para mais explicações:
If you have multiple kernels installed, say linux and linux-lts, by default grub-mkconfig groups them in a submenu. If you do not like this behaviour you can go back to one single menu by adding the following line to
/etc/default/grub:
GRUB_DISABLE_SUBMENU=y