Kernel flavours differ on secondary arches (and there's a bunch
of platform-specific kernels for ARM systems), so let's provide
some more flexible scheme than hardwiring un-def into images.

This might be further enhanced to e.g. set KFLAVOURS to
$$(KFLAVOURS_LATEST)) with it being preset to un-def
where available so rpi images could set it to rpi-un,
and still get reasonable starterkits for free.

The corresponding commit in RELENG-e2k-p9-starterkits
private branch just tore un-def apart => unmergeable.