Makefile: Fake external blob with BINMAN_ALLOW_MISSING=1

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")