sandbox: init cli for -c

sandbox crashes if a variable is set in the -c command, because
hush's top_vars is not allocated.  Call cli_init() from sandbox
to ensure this is done before we execute the -c command.

 $ ./u-boot -c 'a=1'
 ...
 Segmentation fault (core dumped)

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Simon Glass <sjg@chromium.org)>
Tested-by: Simon Glass <sjg@chromium.org)>