mvebu: Fix sysupgrade for GL.iNet GL-MV1000

mvebu: Fix sysupgrade for GL.iNet GL-MV1000

The GL.iNet GL-MV1000 is booting from eMMC and the images for it are in
theory sysupgrade compatible. But the platform upgrade scripts were not
adjusted to select the mmcblock device as upgrade target. This resulted in
a failed sysupgrade because the mtd device (NOR flash) was instead tried to
be modified by the sysupgrade script.

Fixes: 050c24f05c85 ("mvebu: add support for GL.iNet GL-MV1000")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
(cherry picked from commit 07e5e03711d55f94db738446ef9eddc8163b53a6)