sunxi/nand: change BLOCK_SIZE in mksunxiboot to match NAND block size

This change is necessary to calculate correct checksum for NAND
boot. Works both for MMC and NAND. Without it BROM rejects boot image
as invalid (bad checksum). (Changes block size from 0x200 to 0x2000).

Signed-off-by: Daniel Kochmański <dkochmanski@turtle-solutions.eu>
Signed-off-by: Roy Spliet <r.spliet@ultimaker.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>