Prefer CPU microcodes from backports.

They are usually kept pretty much up-to-date there, no need to fetch them
from sid.

config/chroot_apt/preferences

@@ -7,6 +7,10 @@ Package: aircrack-ng
 Pin: release o=Debian,a=unstable
 Pin-Priority: 1010
 
+Package: amd64-microcode
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
 Package: b43-fwcutter
 Pin: release o=Debian,a=unstable
 Pin-Priority: 999
@@ -76,6 +80,10 @@ Package: initramfs-tools
 Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
+Package: intel-microcode
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
 Package: klibc-utils
 Pin: release o=Debian,n=wheezy
 Pin-Priority: 999