ath79: re-enable image generation for GL-AR750S

ath79: re-enable image generation for GL-AR750S

The bootloader only writes the first 2MB of the image to the NOR flash
when installing the NAND factory image. The bootloader is capable of
booting larger kernels as it boots from the memory mapped SPI flash.

Disable the NAND factory image. The NAND can be bootstrapped by writing
the NAND initramfs image using the NOR upgrade method in the bootloader
web-recovery and sysupgrading from there. The NOR variant is not
affected.

Also refactor the partition definitions in the DTS to make them less
annoying to read.

Signed-off-by: David Bauer <mail@david-bauer.net>