Kconfig: set default BUILD_TARGET for kirkwood

Now that BUILD_TARGET is in Kconfig we can define a default for boards
using the Kirkwood SoC.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Cc: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/Kconfig b/Kconfig
index ad32c1d..2a48f53 100644
--- a/Kconfig
+++ b/Kconfig
@@ -230,6 +230,7 @@
 	default "u-boot-spl.kwb" if ARCH_MVEBU && SPL_BUILD
 	default "u-boot-elf.srec" if RCAR_GEN3
 	default "u-boot.itb" if SPL_LOAD_FIT && ARCH_SUNXI
+	default "u-boot.kwb" if KIRKWOOD
 	help
 	  Some SoCs need special image types (e.g. U-Boot binary
 	  with a special header) as build targets. By defining