Bluetooth: controller: Introduce ULL LLL architecture

This is a squash merge of commits introducing the new split
Upper Link Layer and Lower Link Layer architecture of the
Bluetooth Low Energy controller.

This introduces a new, improved Link Layer based on the
concept of split responsibilities; The Upper Link Layer
(ULL) is in charge of control procedures, inter-event
scheduling and overall role management. The code for the
ULL is shared among all hardware implementations. The
Lower Link Layer (LLL) is responsible for the intra-event
scheduling and vendor specific radio hardware access.

The communication between ULL and LLL is achieved through
a set of FIFOs that contain both control and data packets.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>
Signed-off-by: Wolfgang Puffitsch <wopu@oticon.com>
Signed-off-by: Morten Priess <mtpr@oticon.com>