sandbox: Init the EC properly even if no state file is available

This currently relies on sandbox attempting to read a state file. At
present it always does, even when there is no state file, in which case it
fails, but still inits the EC.

That is a bug, so update this driver to set the current image always, even
if no state is read.

Signed-off-by: Simon Glass <sjg@chromium.org>