imx.mk: introduce MKIMAGE_DT variable

The dtb name used by mkimage isn't necessarily related to the the
U-Boot dtb name. e.g. imx8mp-verdin is using imx8mp-verdin-wifi-dev.dtb

Introduce a variable to allow per board setting.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/imx.mk b/imx.mk
index bf4faff..515bffd 100644
--- a/imx.mk
+++ b/imx.mk
@@ -13,6 +13,7 @@
 U_BOOT_DEFCONFIG  ?= imx8mq_evk_defconfig
 U_BOOT_DT         ?= imx8mq-evk.dtb
 LINUX_DT          ?= imx8mq-evk.dtb
+MKIMAGE_DT        ?= fsl-imx8mq-evk.dtb
 MKIMAGE_SOC       ?= iMX8MQ
 
 BR2_TARGET_GENERIC_GETTY_PORT ?= ttymxc0
@@ -161,7 +162,7 @@
 	ln -sf $(UBOOT_PATH)/u-boot-nodtb.bin $(MKIMAGE_SOC_PATH)/
 	ln -sf $(UBOOT_PATH)/spl/u-boot-spl.bin $(MKIMAGE_SOC_PATH)/
 	ln -sf $(UBOOT_PATH)/arch/arm/dts/$(U_BOOT_DT) \
-		$(MKIMAGE_SOC_PATH)/fsl-$(U_BOOT_DT)
+		$(MKIMAGE_SOC_PATH)/$(MKIMAGE_DT)
 	ln -sf $(UBOOT_PATH)/tools/mkimage $(MKIMAGE_SOC_PATH)/mkimage_uboot
 	$(MAKE) -C $(MKIMAGE_PATH) SOC=$(MKIMAGE_SOC) flash_spl_uboot
 #> +If you want to run with HDMI, copy signed_hdmi_imx8m.bin to imx-mkimage/iMX8M