Bluetooth: Implement the Add Advertising command

This patch adds the most basic implementation for the
"Add Advertisement" command. All state updates between the
various HCI settings (POWERED, ADVERTISING, ADVERTISING_INSTANCE,
and LE_ENABLED) has been implemented. The command currently
supports only setting the advertising data fields, with no flags
and no scan response data.

Signed-off-by: Arman Uguray <armansito@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>