sunxi: Increase SPL max size to 0x7600 and move u-boot.img start to 40KB offset

Turns out we actually have quite a bit more than 24KB available for SPL,
and with Falcon mode enabled we need that space.
sunxi: Automatically build u-boot-sunxi-with-spl.bin

also update u-boot-sunxi-with-spl.bin rule to use u-boot.img instead
of u-boot.bin.