ipq40xx: fix openmesh sysupgrade with tar content out of order

ipq40xx: fix openmesh sysupgrade with tar content out of order

The tar extraction depends on the order in which the files
are added to the tar file. Since the order is not guaranteed
and depends on the host system, the combined mtd write fails
with sysupgrade images built on some systems.
Fix by changing to tar file order independent mtd write.

Fixes: 86e18f6706e1 ("ipq806x: add support for OpenMesh A42")
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>