Supoport compressed squashfs for kernel (#1418) (#1451)

* Add squashfs with LZ4 and LZO compression to Barebox

* Add squashfs with LZO compression to U-Boot

* Use "raw" as type for kernel partition which is compatible with ext4
  image as well as squashfs.

Note: This does not use squashfs for the kernel but merly adds support
for it. This will allow to downgrade/fallback to the non-squashfs
version. Using squashfs for the kernel partition is implemented in dev
(for release 7).

buildroot-external/scripts/rauc.sh

@@ -43,7 +43,7 @@ function _write_rauc_system() {
     (
         echo "[slot.kernel.${slot_num}]"
         echo "device=/dev/disk/by-partlabel/hassos-kernel${slot_num}"
-        echo "type=ext4"
+        echo "type=raw"
         echo "bootname=${slot_name}"
 
         echo "[slot.rootfs.${slot_num}]"