boards: 96b_carbon: Require BT for NET_L2_BT
boards: 96b_carbon: Require BT for NET_L2_BT

Since NET_L2_BT no longer "select"s BT and the rest of required BT
features, enable them within the defconfig file.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

nashif/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyrproject-rtos/zephyr