Adding a menu entry on GRUB.

[William <willbilly@fedora.email>]

Hello.

I'm currently trying to add a menu entry on GRUB through the Guix
system declaration to boot another GNU+Linux system but I don't have
much clue how to do it with this setup.

The other system currently has three partitions, one for "/boot",
one for "/" and one for "/home", I tried defining a menu entry like
this:

```
(menu-entry
                     (label "GNU/Linux")
                     (device (uuid
                     "d757ca6e-ae43-41b7-bc73-7e80cd270e05" 'ext4)) 
                     (linux "/vmlinuz-linux")
                     (linux-arguments
                     '("root=UUID=da4c92d2-099f-4046-ab1d-1695860f33f0
                     rw net.ifnames=0 quiet")) 
                     (initrd "/intel-ucode.img /amd-ucode.img
                     /initramfs-linux.img"))
```

As you can see, it contains two different UUIDs, this is intended, as
one UUID is for the "/boot" partition, the other one is for the "/"
partition, I copied these values from the grub.cfg menu entry generated
by os-prober during the installation of this system, and they look the
same on the menu entry defined by Guix. 

However, this entry does not work, when I try to boot the system
from the GRUB menu, it says it couldn't find the device by UUID
"d757ca6e-ae43-41b7-bc73-7e80cd270e05", which is the "/boot" partition.

I checked and double checked if they're the right UUIDs and they are,
so the issue seems that GRUB must be aware first that this "/boot"
partition relies on the "/" partition, but on the manual I don't see
exactly how I could define that. 

Is this implemented and possible? When I tried defining a menu entry
for another operating system it did work, but that install was a single
partition.



Both GuixSD and the other operating system are installed on MBR
partitions and use the old "legacy/BIOS" bootstrap method.