Add -s option to config.

This option allows for specifying the directory to use as the location for
kernel source files. This option was ported from NetBSD.

GitHub Pull Request: https://github.com/freebsd/freebsd/pull/18
Submitted by: Steve Kiernan <stevek@juniper.net>, Simon Gerraty <sjg@juniper.net>
Obtained from: Juniper Networks, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D1722
Relnotes: yes