At present sandbox exits when the 'bootm' command completes, since it is not
actually able to run the OS that is loaded. Normally 'bootm' failure is
considered a fatal error in U-Boot.

However this is annoying for tests, which may want to examine the state
after a test is complete. In any case there is a 'reset' command which can
be used to exit, if required.

Change the behaviour to return normally from the 'bootm' command on sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Teddy Reed <teddy.reed@gmail.com>