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

sys/conf/config.mk

@@ -8,6 +8,10 @@
 # the code here when they all produce identical results
 # (or should)
 .if !defined(KERNBUILDDIR)
+opt_global.h:
+.if ${MACHINE} != "mips"
+	@echo "#define VIMAGE 1" > ${.TARGET}
+.endif
 opt_bpf.h:
 	echo "#define DEV_BPF 1" > ${.TARGET}
 .if ${MK_INET_SUPPORT} != "no"

sys/conf/kmod.mk

@@ -106,6 +106,9 @@ NOSTDINC=	-nostdinc
 CFLAGS:=	${CFLAGS:N-I*} ${NOSTDINC} ${INCLMAGIC} ${CFLAGS:M-I*}
 .if defined(KERNBUILDDIR)
 CFLAGS+=	-DHAVE_KERNEL_OPTION_HEADERS -include ${KERNBUILDDIR}/opt_global.h
+.else
+SRCS+=		opt_global.h
+CFLAGS+=	-include ${.OBJDIR}/opt_global.h
 .endif
 
 # Add -I paths for system headers.  Individual module makefiles don't