imx: add rules for U-Boot DTB support

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/Makefile b/Makefile
index 64e8296..4df6118 100644
--- a/Makefile
+++ b/Makefile
@@ -757,6 +757,9 @@
 u-boot-dtb.bin: u-boot.bin dts/dt.dtb FORCE
 	$(call if_changed,cat)
 
+%.imx: %.bin
+	$(Q)$(MAKE) $(build)=arch/arm/imx-common $@
+
 quiet_cmd_copy = COPY    $@
       cmd_copy = cp $< $@
 
@@ -810,9 +813,6 @@
 u-boot.img u-boot.kwb u-boot.pbl: u-boot.bin FORCE
 	$(call if_changed,mkimage)
 
-u-boot.imx: u-boot.bin
-	$(Q)$(MAKE) $(build)=arch/arm/imx-common $@
-
 u-boot.sha1:	u-boot.bin
 		tools/ubsha1 u-boot.bin