apm821xx: build squashfs-rootfs images for the MBL

This patch adds support for squashfs as the root filesystem.

advantages:
 - migrate from a existing -ext4 installation and back
   with the sysupgrade utility
 - existing partition layout will not be lost during switch
 - slightly smaller image size as compared to the -ext4 image.

disadvantages:
 - needs f2fs + tools. This is because fstools rootdisk.c decides based
   on the partition size (currently root partitions > 100 MiB) f2fs is
   used as the rootfs_data filesystem.
 - rootfs_data is placed into the rootfs partition after the squashfs.
   This makes it difficult for tools that expect a /dev/sda${X} device.
   It also makes it difficult for data recovery tools as they might not
   expect to find a embedded partition or will be slightly confused.
   ... or will not support f2fs.

For people with existing build configurations: make sure to include mkf2fs
and f2fsck packages into the image. Otherwise the new -squashfs image will
only boot from the ram-overlay.

Note:
All overlay data (configurations/all installed packages/...) will be
placed in inside the rootfs partition (i.e. /dev/sda2) just after the
squashfs image.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

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