When building multiple kernels using KERNCONF, non-existent KERNCONF
files will produce an error and buildkernel will fail. Previously missing
KERNCONF files silently failed giving no indication as to why, only to
subsequently discover during installkernel that the desired kernel was
never built in the first place.

This is a rework of r302865.

Reviewed by: ngie (previous version, r302865)
MFC after: 2 months
Differential Revision: D7167