Makefile: fix out-of-tree building of "u-boot.img" target

Signed-off-by: NWolfgang Denk <wd@denx.de>

Makefile

@@ -313,7 +313,7 @@ $(obj)u-boot.ldr.srec:	$(obj)u-boot.ldr
 		$(OBJCOPY) ${OBJCFLAGS} -O srec $< $@ -I binary
 
 $(obj)u-boot.img:	$(obj)u-boot.bin
-		./tools/mkimage -A $(ARCH) -T firmware -C none \
+		$(obj)tools/mkimage -A $(ARCH) -T firmware -C none \
 		-a $(TEXT_BASE) -e 0 \
 		-n $(shell sed -n -e 's/.*U_BOOT_VERSION//p' $(VERSION_FILE) | \
 			sed -e 's/"[	 ]*$$/ for $(BOARD) board"/') \