tools: kwbimage: Align kwbimage header to proper size

Currently kwbimage header is always aligned to 4096 bytes. But it does not
have to be aligned to such a high value.

The header needs to be just 4-byte aligned, while some image types have
additional alignment restrictions.

This change reduces size of kwbimage binaries by removing extra padding
between header and data part.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>