cmake: Fix cli shared linking against ubox

The 'ubox' library used in 'cli' target is not using the output of the
'find_library' command. This is a problem when the 'ubox' library is not
located on the standard system library locations.

Use the CMake variable 'ubox', output of 'find_library' call that points
to the library found on the system, instead of hardcoding the library's
name to 'ubox'.

Signed-off-by: Andre Castro <andre.castro.sw@gmail.com>