live-build: reconfigure build after jessie-backports moved to Debian archive

Debian Team finally shut down all jessie-backport mirrors after announcing
discontinuation [1] in July 2018. As we heavily rely on Backported Packages
for VyOS 1.2 we need to move the installation mirrors to Debian Archive.

In addition the repo signing keys already expired requiring us to set
  Acquire::Check-Valid-Until=false
when building the ISO.

[1] https://lists.debian.org/debian-backports-announce/2018/07/msg00000.html
[2] https://wiki.debian.org/LTS/FAQ#Who_fixes_security_issues_with_packages_in_jessie-backports.3F

data/defaults.json

@@ -9,7 +9,8 @@
   "kernel_flavor": "amd64-vyos",
   "additional_repositories": [
     "deb http://repo.saltstack.com/apt/debian/8/amd64/2017.7 jessie main",
-    "deb http://repo.powerdns.com/debian jessie-rec-41 main"
+    "deb http://repo.powerdns.com/debian jessie-rec-41 main",
+    "deb http://archive.debian.org/debian/ jessie-backports main"
   ],
   "custom_packages": []
 }

scripts/live-build-config

@@ -54,9 +54,9 @@ lb config noauto \
         --archive-areas "main contrib non-free" \
         --firmware-chroot false \
         --firmware-binary false \
-        --updates true \
+        --updates false \
         --security true \
-        --backports true \
+        --apt-options "--yes -oAcquire::Check-Valid-Until=false" \
         --apt-indices false
         "${@}"
 """