Bluetooth: controller: Enable use of user defined protocols

Implements hooks to implement user protocols in ull.c
A user defined init function can now be called, this code is gated by
the CONFIG_BT_CTLR_USER_EXT define.
A user defined PDU length can now also be defined using the Kconfig
CONFIG_BT_RX_USER_PDU_LEN

Signed-off-by: Asger Munk Nielsen <asmk@oticon.com>