Makefile: Fake external blob with BINMAN_ALLOW_MISSING=1

This flag was lost by a previous change and has never been restored.
Without it, binman cannot fully handle missing blobs which are
themselves inputs to mkimage.

Discussion on this at [1] indicated that this was necessary but the
patch was not updated.

Restore the flag so that all missing blobs are reported.

Link: https://patchwork.ozlabs.org/project/uboot/patch/20221206020336.315465-1-trini@konsulko.com/

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 93685d0dcb9 ("Makefile: With BINMAN_ALLOW_MISSING=1 don't error")
diff --git a/Makefile b/Makefile
index 81b73a1..da742ce 100644
--- a/Makefile
+++ b/Makefile
@@ -1398,7 +1398,8 @@
                 --toolpath $(objtree)/tools \
 		$(if $(BINMAN_VERBOSE),-v$(BINMAN_VERBOSE)) \
 		build -u -d u-boot.dtb -O . -m \
-		--allow-missing $(if $(BINMAN_ALLOW_MISSING),--ignore-missing) \
+		--allow-missing --fake-ext-blobs \
+		$(if $(BINMAN_ALLOW_MISSING),--ignore-missing) \
 		-I . -I $(srctree) -I $(srctree)/board/$(BOARDDIR) \
 		$(foreach f,$(of_list_dirs),-I $(f)) -a of-list=$(of_list) \
 		$(foreach f,$(BINMAN_INDIRS),-I $(f)) \