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/Makefile b/Makefile
index dc04870..1311592 100644
--- a/Makefile
+++ b/Makefile
@@ -839,7 +839,10 @@
 tpl/u-boot-with-tpl.bin: tpl/u-boot-tpl.bin u-boot.bin FORCE
 	$(call if_changed,pad_cat)
 
-u-boot-with-spl.imx u-boot-with-nand-spl.imx: spl/u-boot-spl.bin u-boot.bin
+SPL: spl/u-boot-spl.bin FORCE
+	$(Q)$(MAKE) $(build)=arch/arm/imx-common $@
+
+u-boot-with-spl.imx u-boot-with-nand-spl.imx: SPL u-boot.bin FORCE
 	$(Q)$(MAKE) $(build)=arch/arm/imx-common $@
 
 MKIMAGEFLAGS_u-boot.ubl = -n $(UBL_CONFIG) -T ublimage -e $(CONFIG_SYS_TEXT_BASE)