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/tools/binman/cmdline.py b/tools/binman/cmdline.py
index e73ff78..700e5a2 100644
--- a/tools/binman/cmdline.py
+++ b/tools/binman/cmdline.py
@@ -52,6 +52,8 @@
             help='Configuration file (.dtb) to use')
     build_parser.add_argument('--fake-dtb', action='store_true',
             help='Use fake device tree contents (for testing only)')
+    build_parser.add_argument('--fake-ext-blobs', action='store_true',
+            help='Create fake ext blobs with dummy content (for testing only)')
     build_parser.add_argument('-i', '--image', type=str, action='append',
             help='Image filename to build (if not specified, build all)')
     build_parser.add_argument('-I', '--indir', action='append',