linux-headers: Update to 3.12.32

We can do this because bootstrap-tools contains an unxz program now
(via busybox).

pkgs/stdenv/linux/scripts/unpack-bootstrap-tools.sh

@@ -55,4 +55,9 @@ echo "#! $out/bin/sh" > $out/bin/egrep
 echo "exec $out/bin/grep -E \"\$@\"" >> $out/bin/egrep
 echo "#! $out/bin/sh" > $out/bin/fgrep
 echo "exec $out/bin/grep -F \"\$@\"" >> $out/bin/fgrep
-chmod +x $out/bin/egrep $out/bin/fgrep
+
+# Provide xz (actually only xz -d will work).
+echo "#! $out/bin/sh" > $out/bin/xz
+echo "exec $builder unxz \"\$@\"" >> $out/bin/xz
+
+chmod +x $out/bin/egrep $out/bin/fgrep $out/bin/xz