Add WITH_REPRODUCIBLE_BUILD src.conf(5) knob to disable kernel metadata

The kernel builds reproducibly, except for the time, date, user, and
hostname baked into the kernel (reported at startup and via the
kern.version sysctl for uname).  Add a build knob to disable the
inclusion of this metadata.

Reviewed by:	jhb
MFC after:	1 month
Relnotes:	Yes
Sponsored by:	Reproducible Builds World Summit 2, Berlin
Differential Revision:	https://reviews.freebsd.org/D8809

sys/conf/kern.opts.mk

@@ -47,7 +47,8 @@ __DEFAULT_NO_OPTIONS = \
     EISA \
     EXTRA_TCP_STACKS \
     NAND \
-    OFED
+    OFED \
+    REPRODUCIBLE_BUILD
 
 # Some options are totally broken on some architectures. We disable
 # them. If you need to enable them on an experimental basis, you

sys/conf/kern.post.mk

@@ -357,8 +357,11 @@ config.o env.o hints.o vers.o vnode_if.o:
 config.ln env.ln hints.ln vers.ln vnode_if.ln:
 	${NORMAL_LINT}
 
+.if ${MK_REPRODUCIBLE_BUILD} != "no"
+REPRO_FLAG="-r"
+.endif
 vers.c: $S/conf/newvers.sh $S/sys/param.h ${SYSTEM_DEP}
-	MAKE=${MAKE} sh $S/conf/newvers.sh ${KERN_IDENT}
+	MAKE=${MAKE} sh $S/conf/newvers.sh ${REPRO_FLAG} ${KERN_IDENT}
 
 vnode_if.c: $S/tools/vnode_if.awk $S/kern/vnode_if.src
 	${AWK} -f $S/tools/vnode_if.awk $S/kern/vnode_if.src -c

tools/build/options/WITH_REPRODUCIBLE_BUILD

@@ -0,0 +1,3 @@
+$FreeBSD$
+Set to exclude build metadata (build time, user, host and path) from the
+kernel and uname output.