iopsys-econet: Remove overlay autoformat

Bringup and format of overlay partition should be done manually. Not in upgrade bundle script.
Description added to Readme.md file.
Mtd squashfs variant does use overlay in mtd8 (container) partition.
This solution is temporary.

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