oxnas: copy ubinized images to binary folder

Since r43778 ubinized images are stored inside KDIR. This makes sense
when using an uninized image to generate a factory image.
On oxnas ubinized images were used for sysupgrade before proper NAND
support was implemented in OpenWrt.
For users still on old builds before sysupgrade.tar was introduced,
ubinized image is thus the only option to conveniantly upgrade to a
more recent build.
Copy ubinized image also to BIN_DIR, so it will be easier for users
to find them.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44357 3c298f89-4303-0410-b956-a3cf2f4a3e73