TT#5444 Revert "TT#5444 Debian stretch doesn't have security support (yet)"

This reverts commit ba081beb91.
The Debian stretch security repository exists nowadays (even
if it's empty).

Change-Id: I1a0054ada1de5ac6bc4bce36da9a63aa074455c7

deployment.sh

@@ -1590,17 +1590,10 @@ EOF
 
 # Debian repositories
 deb ${MIRROR} ${DEBIAN_RELEASE} main contrib non-free
+deb ${SEC_MIRROR} ${DEBIAN_RELEASE}-security main contrib non-free
+deb ${MIRROR} ${DEBIAN_RELEASE}-updates main contrib non-free
 EOF
 
-if [ "$DEBIAN_RELEASE" = "stretch" ] ; then
-  echo  "Warning: not enabling security repository for $DEBIAN_RELEASE"
-  logit "Warning: not enabling security repository for $DEBIAN_RELEASE"
-else
-  echo "deb ${SEC_MIRROR} ${DEBIAN_RELEASE}-security main contrib non-free" >> "$TARGET"/etc/apt/sources.list.d/debian.list
-fi
-
-echo "deb ${MIRROR} ${DEBIAN_RELEASE}-updates main contrib non-free" >> "$TARGET"/etc/apt/sources.list.d/debian.list
-
   # support testing rc releases without providing an according installer package ahead
   if [ -n "$AUTOBUILD_RELEASE" ] ; then
     echo "Running installer with sources.list for $DEBIAN_RELEASE + autobuild release-$AUTOBUILD_RELEASE"