cns3xxx: eliminate hardcoded kernel/rootfs partition split

cns3xxx: eliminate hardcoded kernel/rootfs partition split

This changes the sysupgrade format. To support upgrades from the old
firmware to the new one, legacy images are provided. Because of the old
partition split, these have to be specific to the NOR or SPI device.

The new sysupgrade images are suitable for begin put on flash directly,
and they are independent of NOR vs SPI flash variant.

Flashing back to old firmware is supported via using the old full-flash
images instead of the old sysupgrade images.

Signed-off-by: Felix Fietkau <nbd@nbd.name>