Kconfig: Set default BUILD_TARGET for ARCH_MX6 with SPL

Use BUILD_TARGET to automatically build "u-boot-with-spl.imx" on MX6
targets with SPL.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/Kconfig b/Kconfig
index 1f0904f..66b059f 100644
--- a/Kconfig
+++ b/Kconfig
@@ -256,6 +256,7 @@
 				ARCH_SUNXI || RISCV)
 	default "u-boot.kwb" if KIRKWOOD
 	default "u-boot-with-spl.bin" if ARCH_AT91 && SPL_NAND_SUPPORT
+	default "u-boot-with-spl.imx" if ARCH_MX6 && SPL
 	help
 	  Some SoCs need special image types (e.g. U-Boot binary
 	  with a special header) as build targets. By defining