Bluetooth: controller: Introduce BLUETOOTH_LL_SW

In preparation for future hardware implementations of the BLE
Link Layer, this patch introduces the BLUETOOTH_LL_SW
configuration option to specify that the default software
LL is to be built.

Change-Id: I8b9d5b5e0d2926d18f9e8c8f042a74326895bf95
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>