Hello.
As of now, building a system requires that the kernel package includes the lib/modules directory, probably with at least some loadable modules, which is not necessary the case for custom packages. This can be fixed trivially with a check in operating-system-directory-base-entries:
(packages->manifest (append (if (file-exists? #$(file-append kernel "/lib/modules")) (list kernel) '()) modules))
But may be it would be better to make a separate output for the linux-libre modules and make kernel-loadable-modules include it by default.
