Add Partitions to GRUB Menu

1. Install os-prober if it's not already installed.
2. Mount all the EFI partitions (the mount point doesn't matter so long as they're mounted somewhere). Run sudo fdisk -l to see which are the EFI partitions.
3. Run sudo grub-mkconfig -o /boot/grub/grub.cfg.

You should now see new entries in the GRUB menu.