Bluetooth: Host: Settings: Handle extra settings_load before bt_enable

Before this change, enabling CONFIG_BT_SETTINGS and calling
settings_load(), but delaying / not calling bt_enable would trigger an
assertion error due to a timeout. The fault is that the settings load
handler for the Bluetooth host assumes bt_enable has already been called
and sends HCI commands to the controller. This times out if HCI is not
running.

The fix is to skip loading Bluetooth settings before bt_enable. The doc
is updated to guide the user on how to enable Bluetooth after settings
have been loaded before.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>