arm-trusted-firmware: replace mkimage invocation with MKIMAGE

We already use $(MKIMAGE) instead of $(HOST_DIR)/bin/mkimage in xvisor, linux,
and cpio; use it here too.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/arm-trusted-firmware/arm-trusted-firmware.mk

@@ -80,7 +80,7 @@ define ARM_TRUSTED_FIRMWARE_BL31_UBOOT_BUILD
 # Get the entry point address from the elf.
 	BASE_ADDR=$$($(TARGET_READELF) -h $(ARM_TRUSTED_FIRMWARE_IMG_DIR)/bl31/bl31.elf | \
 	             sed -r '/^  Entry point address:\s*(.*)/!d; s//\1/') && \
-	$(HOST_DIR)/bin/mkimage \
+	$(MKIMAGE) \
 		-A arm64 -O arm-trusted-firmware -C none \
 		-a $${BASE_ADDR} -e $${BASE_ADDR} \
 		-d $(ARM_TRUSTED_FIRMWARE_IMG_DIR)/bl31.bin \