apm821xx: split MBL's rootfs.img.gz into a sysupgrade and factory image

apm821xx: split MBL's rootfs.img.gz into a sysupgrade and factory image

@vahid-dan reported a issue with extracting the rpi images with
Gnome's Archive Manager:
"Ubuntu Archive Manager cannot extract the file and it just
throws a general error message: "An error occurred while
extracting files".
<https://forum.lede-project.org/t/corrupted-pre-built-v18-06-0-rc2-image-for-rpi>

The MBL's rootfs.img.gz image is generated in much the same way.
Hence this patch preemptively splits the rootfs.img.gz image into
a sysupgrade and a factory image.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
(backported from cff6548948f76b8d0c922ef5e5d456a88a78ab2e)