Adds -i option that allows specifying a ramdisk file to be added to the
FIT image when we are using the automatic FIT mode (no ITS file).

This makes adding Depthcharge support to LAVA much more convenient, as
no additional configuration files need to be kept around in the machine
that dispatches jobs to the boards.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Matt Hart <matthew.hart@linaro.org>
Cc: Neil Williams <codehelp@debian.org>
Reviewed-by: Simon Glass <sjg@chromium.org>