ARC: AXS10x/HSDK: Use our own version of mkimage

Since U-Boot tools are being built anyways it's much nicer
to use them instead of relying on some of them bein installed
on build host (which might easily not be the case).

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/board/synopsys/hsdk/config.mk b/board/synopsys/hsdk/config.mk
index 16fb59c..9e280f9 100644
--- a/board/synopsys/hsdk/config.mk
+++ b/board/synopsys/hsdk/config.mk
@@ -6,6 +6,6 @@
 	$(Q)python3 $(srctree)/board/$(BOARDDIR)/headerize-hsdk.py \
 		--arc-id 0x52 --image $(srctree)/u-boot.bin \
 		--elf $(srctree)/u-boot
-	$(Q)mkimage -T script -C none -n 'uboot update script' \
+	$(Q)tools/mkimage -T script -C none -n 'uboot update script' \
 		-d $(srctree)/u-boot-update.txt \
 		$(srctree)/u-boot-update.scr &> /dev/null