arm, imx, Makefile: fix u-boot-dtb.imx build in CONFIG_MULTI_DTB_FIT case

in case CONFIG_MULTI_DTB_FIT is set and u-boot-dtb.imx
image is build, currently u-boot-dtb.bin is used for
generating the u-boot-dtb.imx binary, which is wrong, as
it contains only a dtb blob not the fit.blob

Use instead the u-boot-fit-dtb.bin for generating
u-boot-dtb.imx which contains the fit.blob.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/arch/arm/mach-imx/Makefile b/arch/arm/mach-imx/Makefile
index daf20f2..898478f 100644
--- a/arch/arm/mach-imx/Makefile
+++ b/arch/arm/mach-imx/Makefile
@@ -112,7 +112,16 @@
 u-boot.imx: u-boot.bin u-boot.cfgout $(PLUGIN).bin FORCE
 	$(call if_changed,mkimage)
 
-ifeq ($(CONFIG_OF_SEPARATE),y)
+ifeq ($(CONFIG_MULTI_DTB_FIT),y)
+MKIMAGEFLAGS_u-boot-dtb.imx = -n $(filter-out $(PLUGIN).bin $< $(PHONY),$^) \
+	-T $(IMAGE_TYPE) -e $(CONFIG_SYS_TEXT_BASE)
+u-boot-dtb.imx: MKIMAGEOUTPUT = u-boot-dtb.imx.log
+
+u-boot-dtb.imx: u-boot-fit-dtb.bin u-boot-dtb.cfgout $(PLUGIN).bin FORCE
+ifeq ($(DEPFILE_EXISTS),0)
+	$(call if_changed,mkimage)
+endif
+else ifeq ($(CONFIG_OF_SEPARATE),y)
 MKIMAGEFLAGS_u-boot-dtb.imx = -n $(filter-out $(PLUGIN).bin $< $(PHONY),$^) \
 	-T $(IMAGE_TYPE) -e $(CONFIG_SYS_TEXT_BASE)
 u-boot-dtb.imx: MKIMAGEOUTPUT = u-boot-dtb.imx.log