Allow system configuration without a bootloader

[kanichos]

Hello.

I am very happy with manually setting up my boot programs.  On UEFI systems I 
don’t use a standalone bootloader at all, because I think it is so unnecessary, 
and rely on Linux ‘EFI stub’ functionality.  I don’t ever want any automatic 
scripts to intervene.

For a couple of years I have used this:
(bootloader (grub-configuration (target "/dev/null")))
It works, but produces an ugly error during configuration.

I believe the field should actually be made optional.