cmd: mvebu/bubt: Set correct default image name for 32-bit Armada SoCs

32-bit Armada SoCs uses u-boot binary packed in kwbimage format. Name of
the image is in CONFIG_BUILD_TARGET option. So use it as a default option
in Kconfig.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/cmd/mvebu/Kconfig b/cmd/mvebu/Kconfig
index 8f30a0c..9f6ad2d 100644
--- a/cmd/mvebu/Kconfig
+++ b/cmd/mvebu/Kconfig
@@ -60,6 +60,7 @@
 
 config MVEBU_UBOOT_DFLT_NAME
 	string "Default image name for bubt command"
+	default BUILD_TARGET if ARMADA_32BIT && BUILD_TARGET != ""
 	default "flash-image.bin"
 	help
 	  This option should contain a default file name to be used with