buildkernel: always add standard kernel configuration include path

This should change nothing for kernel configurations at the standard
locations in the source tree. However, if KERNCONFDIR is used to
specify a custom location for a kernel configuration file (e.g., out of
tree), then both the custom location and the standard location, in this
order, will be used as include paths for config(8). This will allow the
kernel configuration to include files from both locations.

Reviewed by: bdrewery
MFC after: 16 days
Differential Revision: https://reviews.freebsd.org/D22057