oxnas: add missing NAND partitions

oxnas: add missing NAND partitions

add kernel and other missing NAND flash partitions to allow booting
using stock loader instead of the 2nd-stage bootloader.
This will ease installation from stock firmware on pogoplugs as well
as stg212 aka. medion nas.

Existing users of OpenWrt on those boards will have to boot initramfs
generated after this commit and subsequently flash first the ubinized
image, then again run initramfs to flash sysupgrade.tar image, and
then restore bootcmd in U-Boot environment back to the original setting
('nboot 60500000 0 440000' or something like that) instead of jumping
into the now no longer used 2nd-stage loader.

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