swupdate-image: upgrade to ext4

ext4 is smaller as ext3, resulting in smaller ramdisk.

Signed-off-by: Stefano Babic <sbabic@denx.de>

recipes-extended/images/swupdate-image.bb

@@ -22,7 +22,7 @@ export SYSVINIT = "no"
 LICENSE = "MIT"
 
 IMAGE_CLASSES += " image_types_uboot"
-IMAGE_FSTYPES = "ext3.gz.u-boot"
+IMAGE_FSTYPES = "ext4.gz.u-boot"
 
 IMAGE_ROOTFS_SIZE = "8192"