Compress the ramdisk using xz.

It compresses the same as LZMA that was previously used and is now deprecated.
This needs a 2.6.38+ Debian kernel to work, which this branch will anyway need.

config/chroot_local-hooks/12-initramfs-compress

@@ -6,4 +6,4 @@ OPTS_FILE='/etc/initramfs-tools/initramfs.conf'
 
 [ -f "${OPTS_FILE}" ] || exit 11
 
-sed -i'' 's,^COMPRESS=.*,COMPRESS=lzma,' "${OPTS_FILE}"
+sed -i'' 's,^COMPRESS=.*,COMPRESS=xz,' "${OPTS_FILE}"