Kconfig: fix BUILD_TARGET for ARCH_MVEBU

Commit dc146ca11187 ("Kconfig: Migrate CONFIG_BUILD_TARGET") made the
mvebu default build target depend on CONFIG_SPL_BUILD. Unfortunately,
there is no such Kconfig symbol. Use the CONFIG_SPL symbol instead to
fix that.

Cc: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/Kconfig b/Kconfig
index 2a48f53..512c7be 100644
--- a/Kconfig
+++ b/Kconfig
@@ -227,7 +227,7 @@
 config BUILD_TARGET
 	string "Build target special images"
 	default "u-boot-with-spl.sfp" if ARCH_SOCFPGA
-	default "u-boot-spl.kwb" if ARCH_MVEBU && SPL_BUILD
+	default "u-boot-spl.kwb" if ARCH_MVEBU && SPL
 	default "u-boot-elf.srec" if RCAR_GEN3
 	default "u-boot.itb" if SPL_LOAD_FIT && ARCH_SUNXI
 	default "u-boot.kwb" if KIRKWOOD