Explicitly exclude the iucode-tool package.

The main use of iucode-tool in Debian is to only include in the initramfs the
CPU firmware for the currently running CPU. We want firmware for all CPUs, not
only the one we build on.

config/chroot_apt/preferences

@@ -30,9 +30,10 @@ Package: intel-microcode
 Pin: origin backports.debian.org
 Pin-Priority: 999
 
+Explanation: We want firmware for all CPUs, not only the one we build on.
 Package: iucode-tool
-Pin: origin backports.debian.org
-Pin-Priority: 999
+Pin: release o=Debian
+Pin-Priority: -1
 
 Package: libcairo2
 Pin: origin backports.debian.org