Connect a 6V or 12V solar panel to the VUSB pin and GND on the Boron.
This video is for the Tracker One, but the process is mostly the same. You may need to adjust the maximum charge rate based on the size of your panel, which is described in the video.
Don't do this on the Argon or Photon 2; they don't have a PMIC chip and have different maximum voltages on the VUSB pin!