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>
diff --git a/board/Marvell/rd6281a/config.mk b/board/Marvell/rd6281a/config.mk
index a4ea769..2bd9f79 100644
--- a/board/Marvell/rd6281a/config.mk
+++ b/board/Marvell/rd6281a/config.mk
@@ -23,3 +23,6 @@
 #
 
 TEXT_BASE = 0x00600000
+
+# Kirkwood Boot Image configuration file
+KWD_CONFIG = $(SRCTREE)/board/$(BOARDDIR)/kwbimage.cfg