tools: kwbimage: Align SPI and NAND images to 256 bytes

Writing into SPI NOR and NAND memory can be done only in 256 bytes long
blocks. Align final image size so that when it is burned into SPI NOR or
NAND memory via U-Boot's commands (sf or mtd), we can use the $filesize
variable directly as the length argument.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>