settings: add shell commands for reading settings

If Kconfig SETTINGS_SHELL option is enabled, add the
following shell commands:
* settings list [subtree] - list all settings belonging to
                            the given subtree,
* settings read name - read a setting with the given name
                       and output hex dump of the value.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>