iopsys-econet: Add mtd_squashfs sysupgrade

@sukrus @AGnau @Marek @LukaszK please review.

platform.sh file must be updated before first sysupgrade:
scp ./feeds/targets/iopsys-econet/base-files/lib/upgrade/platform.sh arcee2:/lib/upgrade

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