iop: Disable UBIFS and EXT4 rootfs build by default
UBIFS and EXT4 can still be re-enabled by interested developers, when it is required for development. All production images should use squashfs only. Produce only one image (squashfs) in all cases to reduce confusion. Previously, this had also already been set for some targets,
Signed-off-by: Andreas Gnau andreas.gnau@iopsys.eu