tools/kwbimage.c: Fix generation of binary header

tools/kwbimage.c: Fix generation of binary header

The binary header ends with one lword, defining if another header
follows this one. This additions 4 bytes need to be taken into
account in the generation of the header size. And the complete
4 bytes at the end of this binary header need to get cleared.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Kevin Smith <kevin.smith@elecsyscorp.com>