cli: hush_modern: Add functions to be called from run_command()

run_command() is called internally by the command run and it can also be called
directly from U-Boot code, e.g. to do unit tests.
This commit adds this path to go to modern hush.

Signed-off-by: Francis Laniel <francis.laniel@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>