tools/kwbimage: add DEBUG option

Offset 0x1 in the generated kwb image file is a set of flags, bit 0
enables debug output from the BootROM firmware.  Allow a DEBUG option in
the kwb configuration to request debug output from the BootROM firmware.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>