Initialize GPIOs for EXPOSURE and STANDBY lines of the camera

This keeps these data lines from floating and causing dark lines in the image.

Although I realize this doesn't fix the issue that some other users were having, it did solve my issue.

See my initial investigation: https://groups.google.com/d/msg/px4users/-eBFpNyOoXU/XUpZUa_FDc8J
Example images: https://groups.google.com/d/msg/px4users/-eBFpNyOoXU/23Kuna93yaUJ