cli: hush_modern: Enable variables expansion for modern hush

Enables variables expansion for modern hush, both for local and environment
variables.
So the following commands:
foo=bar
echo $foo
setenv bar foo
echo $bar
leads to "bar" and "foo" being printed on console output.

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