boards: 96b_carbon: Enable NET_L2_BT by default

BLE is the only networking interface on 96b_carbon, so if
CONFIG_NETWORKING is set, automatically use 6lowpan/BLE link
layer. This will allow to make networking apps work "out of the
box" on 96b_carbon, similar to e.g. qemu_x86 (which automatically
uses SLIP) or frdm_k64f (which automatically uses Ethernet).

This also enables NET_L2_BT_ZEP1656, because most 6LoWPAN/BLE samples
in Zephyr currently use that option, and it is required to achieve
working 6LoWPAN/BLE with Linux kernels currently widely accessible
to the end users. E.g. the latest Ubuntu LTS release, 16.04, ships
with 4.4 kernel, and can be upgraded with a special effort to HWE
kernel which is currently at 4.10. NET_L2_BT_ZEP1656 is needed for
both these kernels.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>