sandbox: Add -i option to enter interactive mode

Normally when U-Boot starts with a command (-c option) it quits when the
command completes. Normally this is what is requires, since the test is
likely complete.

Provide an option to jump into the console instead, so that debugging or
other tasks may be performed before quitting.

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