sys/modules: Make use of SRCS.${KERN_OPT}.

kmod.mk appends the value of SRCS.${KERN_OPT} for each defined kernel
option to SRCS. This helper is shorter than appending to SRCS under
explicit checks on KERN_OPTS.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D38738