kmod build: start generating opt_global.h, include it

For untied module builds, we'll generate opt_foo headers if they're included
in SRCS. However, options that would normally be represented in opt_global.h
aren't properly represented.

Start generating opt_global.h with #define VIMAGE for !mips since it's
almost universally a project default and right now kmods must hack it in
themselves in order to be properly compiled for the default kernel. For
example, ^/sys/modules/pf/Makefile

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D23345