shell: backends: add init log level for dummy shell

Dummy shell was always started with log backend enabled with filter
 statically set to INFO level.
AFAIK no use of such log backend can be made, thus causing waste of
 resources.
The new changes keep INFO as the default filter level. In order to
 disable log backend to be attached to the Dummy shell,
 CONFIG_SHELL_DUMMY_INIT_LOG_LEVEL_NONE has to be set.

Signed-off-by: Marco Argiolas <marco.argiolas@ftpsolutions.com.au>