iopsys-econet: Add mtd_squashfs sysupgrade

This flash layout is not supposed to be used in the future. But more
development devices will be delivered with non UBIFS flash chip.
Therefore mtd squashfs sysupgrade is added to feeds.

Signed-off-by: Bartlomiej Grzeskowiak <bartlomiej.grzeskowiak@iopsys.eu>