binman: add support for creating dummy files for external blobs

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>