Kirkwood: rd6281a: Add kwbimage build support

This patch adds kwbimage configuration file
(used by mkimage utility)
to support u-boot.kwb target on rd6281a platform.

To create Kirkwood boot image to be flashed on NAND,
additional parameter u-boot.kwb need to be passed during make.

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>