tools: kwbimage: Don't parse PAYLOAD keyword

tools: kwbimage: Don't parse PAYLOAD keyword

The PAYLOAD keyword does nothing. No code is using it and both mkimage
and kwbimage completely ignore it. It looks like a relict from the past.
The payload image itself can be specified only via -d parameter to
mkimage.

Signed-off-by: Pali Rohár <[email protected]>
Reviewed-by: Marek Behún <[email protected]>
Reviewed-by: Chris Packham <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
Tested-by: Chris Packham <[email protected]>