tools/kwbimage.c: Make sure that the resulting image is 4-byte aligned

tools/kwbimage.c: Make sure that the resulting image is 4-byte aligned

With the dtb added to the main U-Boot image, it can happen, that
the resulting image is not 4-byte aligned. As the dtb tends to
be unaligned. But the image needs to be 4-byte aligned. At least the
Marvell hdrparser tool complains if its unaligned. By returning 1 here
in kwbimage_generate(), called via tparams->vrec_header() in mkimage.c,
mkimage will automatically pad the resulting image to a 4-byte size
if necessary.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Dirk Eibach <eibach@gdsys.de>