Fix APT pinning for Debian backports.

On Wheezy, we cannot rely on "origin backports.debian.org" anymore,
as the backports were merged into the regular archive.

config/chroot_apt/preferences

@@ -4,11 +4,11 @@ Pin: release o=Debian
 Pin-Priority: -1
 
 Package: aircrack-ng
-Pin: origin backports.debian.org
+Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 1010
 
 Package: amd64-microcode
-Pin: origin backports.debian.org
+Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 999
 
 Package: b43-fwcutter
@@ -16,7 +16,7 @@ Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: florence
-Pin: origin backports.debian.org
+Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 999
 
 Package: poedit
@@ -85,7 +85,7 @@ Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: intel-microcode
-Pin: origin backports.debian.org
+Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 999
 
 Package: klibc-utils
@@ -273,7 +273,7 @@ Pin: release o=TorProject,n=wheezy
 Pin-Priority: 990
 
 Package: *
-Pin: origin backports.debian.org
+Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 200
 
 Package: *