bug#40997: [raid] The bootloader can only be installed to a single drive

[maxim . cournoyer]

Using Guix System, the bootloader declaration doesn't currently seem to
support being installed to multiple drives:

--8<---------------cut here---------------start------------->8---
(bootloader (bootloader-configuration
                 (bootloader grub-bootloader)
                 (target "/dev/sda")
                 (terminal-outputs '(console))))
 --8<---------------cut here---------------end--------------->8---

I suggest to change it so that target can be passed a list like
--8<---------------cut here---------------start------------->8---
(bootloader (bootloader-configuration
                 (bootloader grub-bootloader)
                 (target '("/dev/sda" "/dev/sdb"))
                 (terminal-outputs '(console))))
 --8<---------------cut here---------------end--------------->8---

This is useful when trying to boot a degraded RAID array.

Maxim