I modified the 'kernel->boot-label' function in gnu/system.scm to check if the kernel passed to it is a regular package, and if it isn't then it will try to get the package name and version using 'inferior-package-name' and 'inferior-package-version'.

Note: I wasn't sure if the hack at the beginning of this patch was neccessary and I didn't add it: http://git.savannah.gnu.org/cgit/guix.git/commit/?id=811b21fb15d36b06fde994ca7ef5916a9a19f250
```
  (define inferiors-loaded?
    ;; This hack allows us to provide seamless integration for inferior
    ;; packages while not having a hard dependency on (guix inferior).
    (resolve-module '(guix inferior) #f #f #:ensure #f))

  (define (inferior->entry)
    (module-ref (resolve-interface '(guix inferior))
                'inferior-package->manifest-entry))
```

This is the only issue preventing using an inferior package as a kernel, which is very useful as it means you can avoid recompiling a custom kernel when you reconfigure the system with a newer guix revision that changes the kernel package dependencies.