stand: Use SOURCE_DATE_EPOCH in newvers.sh

The newvers.sh script is used to generate vers.c for 10 programs (at
latest count on amd64): 4 EFI loaders, 3 x86 loaders, two userboots,
and one kboot.  When building without MK_REPRODUCIBLE_BUILD this
resulted in 10 different timestamps being compiled into various files.

If SOURCE_DATE_EPOCH is set, use that value rather than the current
date and time.

Reviewed by:	emaste, imp
Sponsored by:	Amazon
Differential Revision:	https://reviews.freebsd.org/D48803

stand/common/newvers.sh

@@ -43,7 +43,15 @@ done
 shift $((OPTIND - 1))
 
 LC_ALL=C; export LC_ALL
-u=${USER-root} h=${HOSTNAME-`hostname`} t=`date`
+u=${USER-root} h=${HOSTNAME-`hostname`}
+if [ -n "$SOURCE_DATE_EPOCH" ]; then
+	if ! t=$(date -ur $SOURCE_DATE_EPOCH 2>/dev/null); then
+		echo "Invalid SOURCE_DATE_EPOCH" >&2
+		exit 1
+	fi
+else
+	t=`date`
+fi
 r=`awk -F: ' /^[0-9]\.[0-9]+:/ { print $1; exit }' $1`
 
 bootprog_info="FreeBSD/${3} ${2}, Revision ${r}\\n"