brcm2708: split sdcard.img.gz into a sysupgrade and factory image

brcm2708: split sdcard.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>

@blogic told me to split the single sdcard.img.gz for the RPi
into a sysupgrade and a factory image for all brcm2708 targets.
The factory images will have no metadata attached, this way
these utilities that can't deal with the attached metadata will
not fail for no reason.

Cc: John Crispin <john@phrozen.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>