mount store in a separated partition

[Reza Alizadeh Majd]

Hi Guix, 

I want to try `/gnu/store` to a separated partition, which has more capacity 
for packages, for this I use following configuration:

--8<---------------cut here---------------start------------->8---
(operating-system
  ...
  (bootloader (bootloader-configuration
                (bootloader grub-bootloader)
                (target "/dev/vda")))
  (file-systems (cons* (file-system
                        (device "/dev/vda1")
                        (mount-point "/")
                        (type "ext4"))
                       (file-system
                        (device "/dev/sda1")
                        (mount-point "/gnu/store")
                        (needed-for-boot? #t)
                        (type "ext4")
                        (create-mount-point? #t))
                      %base-file-systems))
  ...
--8<---------------cut here---------------end--------------->8---

but using this configuration, GRUB theme wont boot correctly and I receive 
following error during boot: 

--8<---------------cut here---------------start------------->8---
error: no such device:
/gnu/store/...-linux-libre-5.1.16/bzImage.
error: file
`/gnu/store/...-linux-libre-5.1.16/bzImage` not found.
error: you need to load the kernel first.

Press any key to continue...
--8<---------------cut here---------------end--------------->8---

could anyone help me on this matter? 

Thanks,
Reza