brcm2708: add squashfs rootfs image

This patch adds a image with squashfs as the root filesystem.
A rootfs_data partition will be generated on the first boot
and placed inside the rootfs partition (just after the squashfs
image).

advantages:
 - it is possible to migrate from an existing -ext4
   installation and back via sysupgrade.
 - existing partition layout will not be lost.
 - slightly smaller image size.
 - support for attendedsysupgrade

disadvantages:
 - needs f2fs + tools as well. This is because fs-tools decides on the
   blocksize of the sdcard. So either f2fs or ext4 can get choosen as
   the rootfs_data filesystem (depends on the size of the root partition).
 - rootfs_data is placed into the rootfs partition. This makes
   it difficult for tools that expect a /dev/mmc0pX device.
   It also makes it difficult for data recovery tools since they
   might not expect to find a embedded partition or will be
   confused.

For people with existing build configurations: make sure to include mkf2fs
and f2fsck package into the image... Otherwise the new -squashfs image will
boot of a ram-overlay and won't keep the configurations after a reboot.

Cc: Álvaro Fernández Rojas <noltari@gmail.com>
Cc: Paul Spooren <spooren@informatik.uni-leipzig.de>
Cc: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Acked-by: Daniel Golle <daniel@makrotopia.org>

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