Use xz compression for squashfs

Changing compression from default (gzip) to xz will reduce rootfs size
from 247.90 Mbytes down to 208.57 Mbytes. The squashfs compression time
increases by about 3 times (from 11 seconds to 33 on my PC), however
this is a one-time image build operation.

Signed-off-by: Petr Fedchenkov <giggsoff@gmail.com>

pkg/mkrootfs-squash/make-rootfs

@@ -44,7 +44,7 @@ IMGFILE=/rootfs.img
   ROOTFS_PART_SIZE_KB=$(( ( ($ROOTFS_PART_SIZE + 1024) / 1024 * 1024) / 1024 ))
   ROOTFS_PART_SECTORS=$(( $ROOTFS_PART_SIZE_KB * 2 ))
 
-  mksquashfs /tmp/rootfs/ $IMGFILE -noappend
+  mksquashfs /tmp/rootfs/ $IMGFILE -noappend -comp xz -no-recovery
 )
 
 $EXPORT_CMD