add scripts/portable_date: wrapper around date for bsd/linux

date -r $unix_timestamp on bsd is equal to date -d @$unix_timestamp on
linux. To support reproducible builds and not loosing every timestamp
it's required to convert a unix timestamp into human readable timestamp

./scripts/portable_date $unix_timstamp +%T

Signed-off-by: Alexander Couzens <[email protected]>

SVN-Revision: 48583

scripts/portable_date.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+case $(uname) in
+	NetBSD|OpenBSD|DragonFly|FreeBSD|Darwin)
+		date -r $1 $2
+		;;
+	*)
+		date -d @$1 $2
+esac
+
+exit $?