KERNBUILDDIR. Come up with some sensible defaults (though listing them
in kmod.mk may be unwise -- we have no easy way to know what are the
best sensible defaults for everything so we just catch the big stuff).
Append SRCS.${opt} for each option in KERN_OPTS to SRCS to allow easy
conditional compilation. Append any notion of KERN_OPTS_EXTRA to the
list of kernel opts.
Differential Revision: https://reviews.freebsd.org/D1530
