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