sandbox: Don't exit when bootm completes

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>