linux/Makefile.in.advanced: don't depend on phony target for mkimage

Stops continous rebuilds.

target/linux/Makefile.in.advanced

@@ -476,7 +476,7 @@ $(PROJECT_BUILD_DIR)/.linux-version: $(LINUX26_KERNEL)
 	echo $(LINUX26_VERSION_PROBED) > $(PROJECT_BUILD_DIR)/.linux-version
 	echo LINUX_MODULES=`$(LINUX26_VERSION_PROBED)`
 
-$(LINUX26_MKIMAGE_DIR)/mkimage:	u-boot
+$(LINUX26_MKIMAGE_DIR)/mkimage:	$(U_BOOT_TOOLS)
 	mkdir -p $(LINUX26_MKIMAGE_DIR)
 	cp $(MKIMAGE) $(LINUX26_MKIMAGE_DIR)