Allow additional "options" files to be specified in kernel config

The "includeoptions" directive can be used to specify an additional
options file to be used.

This is useful in conjunction with the "files" directive for build
environments to be able to add custom files and options.

Add "-v" flag to enable verbose mode. Added some additional error
messages when in verbose mode.

Obtained from: Juniper Networks, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D39540