Allow genup to run without buildkernel installed

[pbrkr]

I'm running Gentoo within a container and using genup to keep everything up-to-date. To update I run 'genup -n' as the kernel is managed by the host system instead of this container guest. However, genup still tries to run 'buildkernel --is-new-kernel-available' which fails unless I have buildkernel installed.

Would it be possible to make this conditional so that buildkernel is not ran at all if '-n' is passed to genup?