Kirkwood: mv88f6281gtw_ge: Add kwbimage build support

This patch adds kwbimage configuration file
(used by mkimage utility)
to support u-boot.kwb target on mv88f6281gtw_ge board.

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

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