oxnas: change file extension of u-boot+initramfs image

The *u-boot-initramfs* images are not ITB/FIT images but are rather
supposed to be loaded to RAM and jumped-into.
To avoid confusion, change the file extension from itb to bin.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45963 3c298f89-4303-0410-b956-a3cf2f4a3e73