Users are confused which image type they should use and there are more
drawbacks than adavantages in using a r/w ubifs rootfs in constrast to
a read-only squashfs rootfs like:
- less available free flash space due to better compression of squashfs
images
- no support for factory reset due to r/w filesystem
- possibility to break failsafe due to r/w filesystem
Therefore, drop support for r/w ubifs rootfs images.
Signed-off-by: Mathias Kresin <dev@kresin.me>