Support simple camera control operations

[envirover]

RadioRoom should support controlling video streaming by supporting the following commands:

• Start video streaming process on MAV_CMD_VIDEO_START_CAPTURE command.
• Kill video streaming process on MAV_CMD_VIDEO_STOP_CAPTURE command.
• Start image capture on MAV_CMD_IMAGE_START_CAPTURE command.
• Stop image capture on MAV_CMD_IMAGE_STOP_CAPTURE command.
• Capture images on MAV_CMD_DO_DIGICAM_CONTROL command.
• Respond to MAV_CMD_REQUEST_CAMERA_CAPTURE_STATUS command with CAMERA_CAPTURE_STATUS message.

radioroom.conf

...
[camera_handler]
# Command executed when vehicle is armed
on_arm=

# Command executed when vehicle is disarmed
on_disarm=

# Command executed when MAV_CMD_VIDEO_START_CAPTURE received
on_video_start_capture=raspivid -o - -t 0 -fps 30 -w 640 -h 480 -b 1500000 | ffmpeg -re -f h264 -i - -vcodec copy -preset ultrafast -tune zerolatency -f flv rtmp://52.52.203.204/live/stream

# Command executed when MAV_CMD_VIDEO_STOP_CAPTURE received
on_video_stop_capture=pkill raspivid

# Command executed when MAV_CMD_IMAGE_START_CAPTURE received
on_image_start_capture=

# Command executed when MAV_CMD_IMAGE_STOP_CAPTURE received
on_image_stop_capture=

# Command executed when MAV_CMD_ON_DIGICAM_CONTROL received
on_do_digicam_control=raspistill -o /tmp/image.jpg

CAMERA_CAPTURE_STATUS.video_status should be set to 1 if video streaming is on.

VIDEO_STREAM_* MAVLink messages better suite for this purpose, but the spec for these messages is not yet finalized.