Add CDC_ENABLED back

I'd like to have an easy way to disable the CDC functionality in the core. However, the CDC_ENABLED flag seems to be remove as part of this release:

After the change there is no easy way to disable the CDC device.

Please consider adding the CDC_ENABLED back; it can save a lot of time and effort for the people who want to get of that part of the core.

The original post is here: CDC_ENABLED no longer available in USBDesc.h (IDE 1.6.7) - Programming Questions - Arduino Forum

Thanks