Since you are going to use wind as your power source, you'll need to know both the direction and strength of the wind. You can do this with an analog wind sensor. Here is an image of a wind sensor that is fairly inexpensive, from www.moderndevices.com:

You can mount it to the mast if you'd like. You can perhaps use a small piece of heavy-duty tape and mount it to the top of the mast. However, as you'll see a bit later, it will be useful to be able to turn the sensor in order to do a complete sensing of the wind, so it is useful to mount the sensor to a servo.

In order for the BeagleBone Black to talk with this device, you'll need to connect it to the GPIO pins. Here is a close-up of the connections that the wind sensor requires:

You'll need a GND and +V connection. The +V pin will be connected to the 5-volt connection of the BeagleBone Black, P9_5 of the GPIO set, and the GND pin will be connected to the ground connection of the BeagleBone Black ADC...