Honor opt_ipsec.h from kernel builds.

To make this simpler, set the default contents of opt_ipsec.h
for standalone modules in sys/conf/config.mk.

Reviewed by:	imp
MFC after:	2 weeks
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D25046

sys/conf/config.mk

@@ -30,6 +30,8 @@ opt_inet.h:
 opt_inet6.h:
 	@echo "#define INET6 1" > ${.TARGET}
 .endif
+opt_ipsec.h:
+	@echo "#define IPSEC_SUPPORT 1" > ${.TARGET}
 .if ${MK_RATELIMIT} != "no"
 opt_ratelimit.h:
 	@echo "#define RATELIMIT 1" > ${.TARGET}

sys/modules/ipsec/Makefile

@@ -8,7 +8,4 @@ SRCS=	if_ipsec.c ipsec.c ipsec_input.c ipsec_mbuf.c ipsec_mod.c \
 	opt_inet.h opt_inet6.h opt_ipsec.h opt_sctp.h 
 SRCS.INET=	udpencap.c
 
-opt_ipsec.h:
-	@echo "#define IPSEC_SUPPORT 1" > ${.TARGET}
-
 .include <bsd.kmod.mk>

sys/modules/tcp/tcpmd5/Makefile

@@ -5,7 +5,4 @@
 KMOD=   tcpmd5
 SRCS=   xform_tcp.c opt_inet.h opt_inet6.h opt_ipsec.h
 
-opt_ipsec.h:
-	@echo "#define IPSEC_SUPPORT 1" > ${.TARGET}
-
 .include <bsd.kmod.mk>