build: fix uImage fake header command

Use a syntax compatible with mkimage from u-boot 2018.03 to fix the
build errors spotted by the build bot.

The images are binary identical to the ones generated with mkimage from
u-boot 2014.10.

Signed-off-by: Mathias Kresin <[email protected]>

include/image-commands.mk

@@ -80,10 +80,11 @@ endef
 # append a fake/empty rootfs uImage header, to fool the bootloaders
 # rootfs integrity check
 define Build/append-uImage-fakeroot-hdr
-	rm -f [email protected]
+	touch [email protected]
 	$(STAGING_DIR_HOST)/bin/mkimage \
 		-A $(LINUX_KARCH) -O linux -T filesystem -C none \
 		-n '$(call toupper,$(LINUX_KARCH)) $(VERSION_DIST) fakeroot' \
+		-d [email protected] \
 		-s \
 		[email protected]
 	cat [email protected] >> $@