kbuild: refactor more IMX image rules

This commit avoids generating ./SPL twice.
  - Fist time descending to spl/
  - Second time as a prerequisite of u-boot-with-spl.imx,
    u-boot-with-nand-spl.imx.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index 8a65c34..17b7408 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -112,3 +112,13 @@
 else
 OBJCOPYFLAGS += -j .text -j .rodata -j .hash -j .data -j .got.plt -j .u_boot_list -j .rel.dyn
 endif
+
+ifneq ($(CONFIG_IMX_CONFIG),)
+ifdef CONFIG_SPL
+ifndef CONFIG_SPL_BUILD
+ALL-y += SPL
+endif
+else
+ALL-y += u-boot.imx
+endif
+endif