Sometimes it is useful to build an image even though external binaries are
not present. This allows the build system to continue to function without
these files, albeit not producing valid images.

U-Boot does with with ATF (ARM Trusted Firmware) today.

Add a new flag to binman to request this behaviour.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>