Bluetooth: Controller: Rename Kconfig to BT_CTLR_ADV_AUX_PDU_BACK2BACK

Rename Kconfig BT_CTLR_ADV_PDU_BACK2BACK to
BT_CTLR_ADV_AUX_PDU_BACK2BACK to represent use of
back-to-back for Extended Advertising PDU chaining support
similar to BT_CTLR_ADV_SYNC_PDU_BACK2BACK that is used to
represent the use of back-to-back for Periodic Advertising
PDU chaining support.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>