binman: add support for creating dummy files for external blobs

While converting to binman for an imx8mq board, it has been found that
building in the u-boot CI fails. This is because an imx8mq requires an
external binary (signed_hdmi_imx8m.bin). If this file cannot be found
mkimage fails.
To be able to build this board in the u-boot CI a binman option
(--fake-ext-blobs) is introduced that can be switched on via the u-boot
makefile option BINMAN_FAKE_EXT_BLOBS. With that the needed dummy files are
created.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index ae9bfab..63d286a 100644
--- a/Makefile
+++ b/Makefile
@@ -1315,6 +1315,7 @@
 		-a tpl-bss-pad=$(if $(CONFIG_TPL_SEPARATE_BSS),,1) \
 		-a spl-dtb=$(CONFIG_SPL_OF_REAL) \
 		-a tpl-dtb=$(CONFIG_TPL_OF_REAL) \
+		$(if $(BINMAN_FAKE_EXT_BLOBS),--fake-ext-blobs) \
 		$(BINMAN_$(@F))
 
 OBJCOPYFLAGS_u-boot.ldr.hex := -I binary -O ihex