mkimage: add debian security updates source when available

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>

contrib/mkimage/debootstrap

@@ -193,7 +193,7 @@ if [ -z "$DONT_TOUCH_SOURCES_LIST" ]; then
 	case "$lsbDist" in
 		debian)
 			# updates and security!
-			if [ "$suite" != 'sid' -a "$suite" != 'unstable' ]; then
+			if curl -o /dev/null -s --head --fail "http://security.debian.org/dists/$suite/updates/main/binary-$(rootfs_chroot dpkg --print-architecture)/Packages.gz"; then
 				(
 					set -x
 					sed -i "