boards: 96b_carbon: support 'make flash' over DFU

Now that dfu-util is a supported flashing method, make it the
default flashing option for Carbon when used via 'make flash'.

Change-Id: I0d2fb9a8cbb4324ea77f1c94ca5df6f1a51e67f6
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>